diff --git a/cmd/fejkdata/main_test.go b/cmd/fejkdata/main_test.go
index ab5bf51..f4dc0af 100644
--- a/cmd/fejkdata/main_test.go
+++ b/cmd/fejkdata/main_test.go
@@ -12,11 +12,48 @@ import (
 	"testing"
 )
 
-const (
-	svSE = "../../data/sv_SE/sv_SE"
-	enUS = "../../data/en_US/en_US"
-	misc = "../../data/misc/misc"
-)
+// svSE, enUS and misc are copies of a shipped module's files without its manifest, so a test
+// layers its categories unprefixed with -d.
+var svSE, enUS, misc string
+
+func TestMain(m *testing.M) {
+	tmp, err := os.MkdirTemp("", "fejkdata-cli-test")
+	if err != nil {
+		panic(err)
+	}
+	svSE, enUS, misc = bareCopy(tmp, "svse"), bareCopy(tmp, "enus"), bareCopy(tmp, "misc")
+	code := m.Run()
+	os.RemoveAll(tmp)
+	os.Exit(code)
+}
+
+// bareCopy copies the category and rows files of the shipped module in data/<module> into a
+// folder under tmp, and returns that folder.
+func bareCopy(tmp, module string) string {
+	src, dst := filepath.Join("..", "..", "data", module), filepath.Join(tmp, module)
+	if err := os.Mkdir(dst, 0o755); err != nil {
+		panic(err)
+	}
+	for _, pattern := range []string{"*.json", "*.tsv"} {
+		files, err := filepath.Glob(filepath.Join(src, pattern))
+		if err != nil {
+			panic(err)
+		}
+		for _, f := range files {
+			if strings.HasPrefix(filepath.Base(f), ".") {
+				continue
+			}
+			b, err := os.ReadFile(f)
+			if err == nil {
+				err = os.WriteFile(filepath.Join(dst, filepath.Base(f)), b, 0o644)
+			}
+			if err != nil {
+				panic(err)
+			}
+		}
+	}
+	return dst
+}
 
 func runOut(stdin string, args ...string) (int, string, string) {
 	var out, errb bytes.Buffer
diff --git a/data/geo/SE/fs_test.go b/data/enus/fs_test.go
similarity index 63%
rename from data/geo/SE/fs_test.go
rename to data/enus/fs_test.go
index 1940ed3..4475144 100644
--- a/data/geo/SE/fs_test.go
+++ b/data/enus/fs_test.go
@@ -1,18 +1,18 @@
-package SE_test
+package enus_test
 
 import (
 	"io/fs"
 	"testing"
 
 	"github.com/larvit/fejkdata"
-	"github.com/larvit/fejkdata/data/geo/SE"
+	"github.com/larvit/fejkdata/data/enus"
 )
 
 func TestFSLoadsByItsManifest(t *testing.T) {
-	if _, err := fs.Stat(SE.FS, ".fejkdata.json"); err != nil {
+	if _, err := fs.Stat(enus.FS, ".fejkdata.json"); err != nil {
 		t.Fatalf("FS holds no manifest: %v", err)
 	}
-	f, err := fejkdata.New(fejkdata.WithDataFS(SE.FS))
+	f, err := fejkdata.New(fejkdata.WithDataFS(enus.FS))
 	if err != nil {
 		t.Fatalf("New(WithDataFS(FS)) = %v", err)
 	}
diff --git a/data/en_US/fs_test.go b/data/geose/fs_test.go
similarity index 63%
rename from data/en_US/fs_test.go
rename to data/geose/fs_test.go
index 6b8c4e7..187af70 100644
--- a/data/en_US/fs_test.go
+++ b/data/geose/fs_test.go
@@ -1,18 +1,18 @@
-package en_US_test
+package geose_test
 
 import (
 	"io/fs"
 	"testing"
 
 	"github.com/larvit/fejkdata"
-	"github.com/larvit/fejkdata/data/en_US"
+	"github.com/larvit/fejkdata/data/geose"
 )
 
 func TestFSLoadsByItsManifest(t *testing.T) {
-	if _, err := fs.Stat(en_US.FS, ".fejkdata.json"); err != nil {
+	if _, err := fs.Stat(geose.FS, ".fejkdata.json"); err != nil {
 		t.Fatalf("FS holds no manifest: %v", err)
 	}
-	f, err := fejkdata.New(fejkdata.WithDataFS(en_US.FS))
+	f, err := fejkdata.New(fejkdata.WithDataFS(geose.FS))
 	if err != nil {
 		t.Fatalf("New(WithDataFS(FS)) = %v", err)
 	}
diff --git a/data/sv_SE/fs_test.go b/data/geous/fs_test.go
similarity index 63%
rename from data/sv_SE/fs_test.go
rename to data/geous/fs_test.go
index 39011b0..afcfe57 100644
--- a/data/sv_SE/fs_test.go
+++ b/data/geous/fs_test.go
@@ -1,18 +1,18 @@
-package sv_SE_test
+package geous_test
 
 import (
 	"io/fs"
 	"testing"
 
 	"github.com/larvit/fejkdata"
-	"github.com/larvit/fejkdata/data/sv_SE"
+	"github.com/larvit/fejkdata/data/geous"
 )
 
 func TestFSLoadsByItsManifest(t *testing.T) {
-	if _, err := fs.Stat(sv_SE.FS, ".fejkdata.json"); err != nil {
+	if _, err := fs.Stat(geous.FS, ".fejkdata.json"); err != nil {
 		t.Fatalf("FS holds no manifest: %v", err)
 	}
-	f, err := fejkdata.New(fejkdata.WithDataFS(sv_SE.FS))
+	f, err := fejkdata.New(fejkdata.WithDataFS(geous.FS))
 	if err != nil {
 		t.Fatalf("New(WithDataFS(FS)) = %v", err)
 	}
diff --git a/data/geo/US/fs_test.go b/data/svse/fs_test.go
similarity index 63%
rename from data/geo/US/fs_test.go
rename to data/svse/fs_test.go
index ca1236e..81673ec 100644
--- a/data/geo/US/fs_test.go
+++ b/data/svse/fs_test.go
@@ -1,18 +1,18 @@
-package US_test
+package svse_test
 
 import (
 	"io/fs"
 	"testing"
 
 	"github.com/larvit/fejkdata"
-	"github.com/larvit/fejkdata/data/geo/US"
+	"github.com/larvit/fejkdata/data/svse"
 )
 
 func TestFSLoadsByItsManifest(t *testing.T) {
-	if _, err := fs.Stat(US.FS, ".fejkdata.json"); err != nil {
+	if _, err := fs.Stat(svse.FS, ".fejkdata.json"); err != nil {
 		t.Fatalf("FS holds no manifest: %v", err)
 	}
-	f, err := fejkdata.New(fejkdata.WithDataFS(US.FS))
+	f, err := fejkdata.New(fejkdata.WithDataFS(svse.FS))
 	if err != nil {
 		t.Fatalf("New(WithDataFS(FS)) = %v", err)
 	}
diff --git a/data_test.go b/data_test.go
index c9e61ba..c21ac6e 100644
--- a/data_test.go
+++ b/data_test.go
@@ -67,7 +67,7 @@ func TestShippedDataCategories(t *testing.T) {
 // v4 UUID (version/variant nibbles fixed), a MAC address, and credit-card numbers
 // whose trailing {luhn()} check passes.
 func TestShippedMiscCategories(t *testing.T) {
-	f := newGenerator(t, "data/misc/misc", WithSeed(1))
+	f := newBareModule(t, "misc", WithSeed(1))
 	v4 := regexp.MustCompile(`^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$`)
 	mac := regexp.MustCompile(`^([0-9a-f]{2}:){5}[0-9a-f]{2}$`)
 	objectid := regexp.MustCompile(`^[0-9a-f]{24}$`)
@@ -95,7 +95,7 @@ func TestShippedMiscCategories(t *testing.T) {
 // codes follow their standard shape, dotted sub-paths resolve, emoji is a real
 // glyph, and a coordinate parses to a point within geographic bounds.
 func TestShippedMiscReferenceData(t *testing.T) {
-	f := newGenerator(t, "data/misc/misc", WithSeed(1))
+	f := newBareModule(t, "misc", WithSeed(1))
 	re := map[string]*regexp.Regexp{
 		"currency":              regexp.MustCompile(`^[A-Z]{3}$`),
 		"currency.name":         regexp.MustCompile(`\p{L}`),
@@ -379,7 +379,7 @@ func TestShippedAddressIsOneNamedPick(t *testing.T) {
 // one locality holds, the region empty for SE, whose address carries none.
 func geoPlaces(t *testing.T, country string) map[string]bool {
 	rows := func(table string) [][]string {
-		b, err := os.ReadFile("data/geo/" + country + "/geo/" + country + "/" + table + ".tsv")
+		b, err := os.ReadFile("data/geo" + strings.ToLower(country) + "/" + table + ".tsv")
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -468,7 +468,7 @@ func TestShippedTableCellsAreTrimmed(t *testing.T) {
 }
 
 // TestShippedNamespacedTree loads every shipped module and reaches each locale through its
-// folder segment: data/sv_SE/sv_SE/person.json -> sv_SE.person.
+// namespace: data/svse/person.json, mounted under sv_SE, is sv_SE.person.
 func TestShippedNamespacedTree(t *testing.T) {
 	f := shipped(t, WithSeed(1))
 	for _, path := range []string{"sv_SE.person", "en_US.person", "sv_SE.address.locality"} {
diff --git a/fejkdata_test.go b/fejkdata_test.go
index b680b35..3c2751a 100644
--- a/fejkdata_test.go
+++ b/fejkdata_test.go
@@ -3,6 +3,8 @@ package fejkdata
 import (
 	"errors"
 	"io/fs"
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 	"testing/fstest"
@@ -35,6 +37,17 @@ func newGeneratorN(t *testing.T, dirs []string, opts ...Option) *Generator {
 	return f
 }
 
+// newBareModule loads the shipped module in data/<module> without its manifest, so its
+// categories sit at the root, failing on error.
+func newBareModule(t *testing.T, module string, opts ...Option) *Generator {
+	t.Helper()
+	f, err := New(append([]Option{WithDataFS(withoutManifest{os.DirFS(filepath.Join("data", module))})}, opts...)...)
+	if err != nil {
+		t.Fatalf("New(%s without its manifest): %v", module, err)
+	}
+	return f
+}
+
 // fake generates a value, failing the test on error.
 func fake(t *testing.T, f *Generator, path string) string {
 	t.Helper()
diff --git a/imports_test.go b/imports_test.go
index 1842f26..a43043b 100644
--- a/imports_test.go
+++ b/imports_test.go
@@ -31,12 +31,12 @@ func TestPackageImports(t *testing.T) {
 	allowed := map[string][]string{
 		".":                    {"internal/builtinfunc", "internal/datafiles", "internal/datatype", "internal/drawstate", "internal/grammar", "internal/invariant", "internal/jsonvalue", "internal/proven", "internal/rows"},
 		"cmd/fejkdata":         {".", "data"},
-		"data":                 {"data/en_US", "data/geo/SE", "data/geo/US", "data/misc", "data/sv_SE"},
-		"data/en_US":           nil,
-		"data/geo/SE":          nil,
-		"data/geo/US":          nil,
+		"data":                 {"data/enus", "data/geose", "data/geous", "data/misc", "data/svse"},
+		"data/enus":            nil,
+		"data/geose":           nil,
+		"data/geous":           nil,
 		"data/misc":            nil,
-		"data/sv_SE":           nil,
+		"data/svse":            nil,
 		"internal/builtinfunc": {"internal/datatype", "internal/drawstate", "internal/grammar", "internal/invariant", "internal/proven"},
 		"internal/datafiles":   {"internal/grammar", "internal/jsonvalue"},
 		"internal/datatype":    nil,
@@ -48,13 +48,13 @@ func TestPackageImports(t *testing.T) {
 		"internal/rows":        {"internal/drawstate", "internal/grammar", "internal/invariant"},
 	}
 	testsAlso := map[string][]string{
-		".":            {"data", "data/en_US", "data/geo/SE", "data/geo/US", "data/misc", "data/sv_SE"},
+		".":            {"data", "data/enus", "data/geose", "data/geous", "data/misc", "data/svse"},
 		"cmd/fejkdata": {"internal/grammar"},
-		"data/en_US":   {"."},
-		"data/geo/SE":  {"."},
-		"data/geo/US":  {"."},
+		"data/enus":    {"."},
+		"data/geose":   {"."},
+		"data/geous":   {"."},
 		"data/misc":    {"."},
-		"data/sv_SE":   {"."},
+		"data/svse":    {"."},
 	}
 	seen := map[string]bool{}
 	eachPackage(t, func(dir string, pkg *build.Package) {
diff --git a/reference_test.go b/reference_test.go
index 8a323d0..7b04e3e 100644
--- a/reference_test.go
+++ b/reference_test.go
@@ -206,7 +206,7 @@ func TestNewErrorPathIsCanonical(t *testing.T) {
 		{
 			"bad reference reached through another reference",
 			map[string]string{"a": `"{/b}"`, "b": `"{/nope}"`},
-			`fejkdata: b: reference {/nope}: no entry "nope", and no loaded module names a module it reads by default`,
+			`fejkdata: b: reference {/nope}: no entry "nope", and no loaded manifest names a module to add`,
 		},
 	}
 	for _, c := range cases {
diff --git a/shipped_test.go b/shipped_test.go
index 7ab659b..37b3a59 100644
--- a/shipped_test.go
+++ b/shipped_test.go
@@ -18,11 +18,11 @@ import (
 	"testing/fstest"
 
 	"github.com/larvit/fejkdata/data"
-	"github.com/larvit/fejkdata/data/en_US"
-	"github.com/larvit/fejkdata/data/geo/SE"
-	"github.com/larvit/fejkdata/data/geo/US"
+	"github.com/larvit/fejkdata/data/enus"
+	"github.com/larvit/fejkdata/data/geose"
+	"github.com/larvit/fejkdata/data/geous"
 	"github.com/larvit/fejkdata/data/misc"
-	"github.com/larvit/fejkdata/data/sv_SE"
+	"github.com/larvit/fejkdata/data/svse"
 	"github.com/larvit/fejkdata/internal/datafiles"
 )
 
@@ -36,24 +36,42 @@ func (w withoutManifest) Open(name string) (fs.File, error) {
 	return w.FS.Open(name)
 }
 
-// shippedModule is a module data.Modules lists, and its folder under data/, slash-separated.
+// shippedModule is a module data.Modules lists, its folder under data/, and the namespace its
+// manifest mounts it under.
 type shippedModule struct {
-	fsys fs.FS
-	dir  string
+	fsys      fs.FS
+	dir       string
+	namespace string
 }
 
 // importPath is the Go package the module's FS is exported from.
 func (m shippedModule) importPath() string { return "github.com/larvit/fejkdata/data/" + m.dir }
 
-// holds reports whether the category at path is the module's: its tree spells its folder.
-func (m shippedModule) holds(path string) bool {
-	return strings.HasPrefix(path, strings.ReplaceAll(m.dir, "/", ".")+".")
+// holds reports whether the category at path is the module's.
+func (m shippedModule) holds(path string) bool { return strings.HasPrefix(path, m.namespace+".") }
+
+// whole is the module's files with a manifest naming only its namespace, so it loads whole in New.
+func (m shippedModule) whole(t testing.TB) fs.FS {
+	t.Helper()
+	out := fstest.MapFS{datafiles.ManifestFile: {Data: []byte(`{"namespace": "` + m.namespace + `"}`)}}
+	err := fs.WalkDir(m.fsys, ".", func(p string, d fs.DirEntry, err error) error {
+		if err != nil || d.IsDir() || p == datafiles.ManifestFile {
+			return err
+		}
+		b, err := fs.ReadFile(m.fsys, p)
+		out[p] = &fstest.MapFile{Data: b}
+		return err
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	return out
 }
 
 // shippedModules is every module data.Modules lists, each beside its folder under data/.
 func shippedModules(t testing.TB) []shippedModule {
 	t.Helper()
-	out := []shippedModule{{en_US.FS, "en_US"}, {SE.FS, "geo/SE"}, {US.FS, "geo/US"}, {misc.FS, "misc"}, {sv_SE.FS, "sv_SE"}}
+	out := []shippedModule{{enus.FS, "enus", "en_US"}, {geose.FS, "geose", "geo.SE"}, {geous.FS, "geous", "geo.US"}, {misc.FS, "misc", "misc"}, {svse.FS, "svse", "sv_SE"}}
 	var listed []fs.FS
 	for _, m := range out {
 		listed = append(listed, m.fsys)
@@ -70,7 +88,7 @@ func newShippedWhole(t testing.TB, opts ...Option) *Generator {
 	t.Helper()
 	var modules []fs.FS
 	for _, m := range shippedModules(t) {
-		modules = append(modules, withoutManifest{m.fsys})
+		modules = append(modules, m.whole(t))
 	}
 	f, err := New(append([]Option{WithDataFS(modules...)}, opts...)...)
 	if err != nil {
@@ -141,7 +159,8 @@ func shippedManifests(whole *Generator, modules []shippedModule) ([][]byte, erro
 	out := make([][]byte, len(modules))
 	for i := range modules {
 		var b bytes.Buffer
-		b.WriteString("{\n\t\"index\": {" + strings.Join(index[i], ",") + "\n\t}")
+		namespace, _ := json.Marshal(modules[i].namespace)
+		b.WriteString("{\n\t\"index\": {" + strings.Join(index[i], ",") + "\n\t},\n\t\"namespace\": " + string(namespace))
 		if reads := readsOf(i, direct, modules); len(reads) > 0 {
 			line, _ := json.Marshal(reads)
 			b.WriteString(",\n\t\"reads\": " + string(line))
@@ -211,11 +230,11 @@ func TestEachShippedModuleRendersBesideTheModulesItReads(t *testing.T) {
 }
 
 func TestAReadNoModuleProvidesNamesTheReadersModules(t *testing.T) {
-	f, err := New(WithDataFS(sv_SE.FS))
+	f, err := New(WithDataFS(svse.FS))
 	if err != nil {
-		t.Fatalf("New(WithDataFS(sv_SE.FS)) = %v", err)
+		t.Fatalf("New(WithDataFS(svse.FS)) = %v", err)
 	}
-	if _, err := f.Fake("sv_SE.address"); err == nil || !strings.Contains(err.Error(), "the loaded modules read by default github.com/larvit/fejkdata/data/geo/SE") {
+	if _, err := f.Fake("sv_SE.address"); err == nil || !strings.Contains(err.Error(), "is a module missing? The loaded modules read github.com/larvit/fejkdata/data/geose") {
 		t.Errorf(`Fake("sv_SE.address") = %v, want the module it reads named`, err)
 	}
 	for name, opt := range map[string]Option{
@@ -226,7 +245,7 @@ func TestAReadNoModuleProvidesNamesTheReadersModules(t *testing.T) {
 		if err == nil {
 			_, err = f.Fake("x")
 		}
-		if err == nil || !strings.Contains(err.Error(), "no loaded module names a module it reads by default") {
+		if err == nil || !strings.Contains(err.Error(), "no loaded manifest names a module to add") {
 			t.Errorf("%s: x = %v, want it said that no loaded module names one", name, err)
 		}
 	}
@@ -237,7 +256,7 @@ func TestADataPathManifestNamesWhatItReads(t *testing.T) {
 		".fejkdata.json": `{"reads": ["example.com/places"]}`,
 		"x.json":         `"{/place}"`,
 	})
-	if _, err := New(WithDataPath(dir)); err == nil || !strings.Contains(err.Error(), "the loaded modules read by default example.com/places") {
+	if _, err := New(WithDataPath(dir)); err == nil || !strings.Contains(err.Error(), "is a module missing? The loaded modules read example.com/places") {
 		t.Errorf("New = %v, want the module x reads named", err)
 	}
 }
@@ -250,8 +269,43 @@ func TestAnIndexedModuleNamesWhatItReads(t *testing.T) {
 	if err != nil {
 		t.Fatalf("New = %v", err)
 	}
-	if _, err := f.Fake("x"); err == nil || !strings.Contains(err.Error(), "the loaded modules read by default example.com/places") {
-		t.Errorf(`Fake("x") = %v, want the module x reads named`, err)
+	const note = "is a module missing? The loaded modules read example.com/places"
+	for name, reach := range map[string]func() error{
+		"Fake":         func() error { _, err := f.Fake("x"); return err },
+		"FakeTemplate": func() error { _, err := f.FakeTemplate("{/x}"); return err },
+		"FakeStruct": func() error {
+			return f.FakeStruct(&struct {
+				A string `fake:"/x"`
+			}{})
+		},
+	} {
+		if err := reach(); err == nil || strings.Count(err.Error(), note) != 1 {
+			t.Errorf("%s reaching x = %v, want the module x reads named once", name, err)
+		}
+	}
+	if _, err := f.FakeTemplate("{/nope}"); err == nil || strings.Contains(err.Error(), "module") {
+		t.Errorf("FakeTemplate({/nope}) = %v, want no module named: no load failed", err)
+	}
+}
+
+func TestANamespaceMountsAModuleUnderIt(t *testing.T) {
+	for name, opt := range map[string]Option{
+		"a data path": WithDataPath(writeFiles(t, map[string]string{
+			".fejkdata.json": `{"namespace": "my.data"}`,
+			"word.json":      `"hej"`,
+		})),
+		"an indexed source": WithDataFS(fstest.MapFS{
+			".fejkdata.json": {Data: []byte(`{"index": {"my.data.word": {"paths": [""]}}, "namespace": "my.data"}`)},
+			"word.json":      {Data: []byte(`"hej"`)},
+		}),
+	} {
+		f, err := New(opt)
+		if err != nil {
+			t.Fatalf("New(%s) = %v", name, err)
+		}
+		if got := fake(t, f, "my.data.word"); got != "hej" {
+			t.Errorf("%s: my.data.word = %q, want hej", name, got)
+		}
 	}
 }
 
@@ -427,6 +481,9 @@ func TestABrokenManifestFailsNewNamingEveryMistake(t *testing.T) {
 		`{"reads": "b"}`:        {"reads must be a list, not a string"},
 		`{"reads": ["", 3]}`:    {"reads item 1 is empty", "reads item 2 must be a string, not a number"},
 		`{"reads": ["a", "a"]}`: {`reads item 2 repeats "a"`},
+		`{"namespace": 5}`:      {"namespace must be a string, not a number"},
+		`{"namespace": "a..b"}`: {`namespace "a..b"`},
+		`{"index": {"b.x": {"paths": [""]}}, "namespace": "a"}`: {`index entry "b.x" lies outside the namespace "a"`},
 	} {
 		_, err := New(WithDataFS(fstest.MapFS{".fejkdata.json": {Data: []byte(manifest)}}))
 		for _, w := range want {
diff --git a/table_test.go b/table_test.go
index c704704..8be4f86 100644
--- a/table_test.go
+++ b/table_test.go
@@ -778,7 +778,7 @@ func TestTableOverridesByLayering(t *testing.T) {
 }
 
 func TestShippedTables(t *testing.T) {
-	f := newGenerator(t, "data/misc/misc", WithSeed(1))
+	f := newBareModule(t, "misc", WithSeed(1))
 	for path, want := range map[string]string{
 		"territory[SE]":                         "Sweden",
 		"territory[Sweden].alpha3":              "SWE",
@@ -992,7 +992,7 @@ func TestEveryTldRowSpellsItsKey(t *testing.T) {
 // columns asked for and that every row fills them.
 func shippedRows(t *testing.T, file string, want ...string) (map[string]int, [][]string) {
 	t.Helper()
-	data, err := os.ReadFile(filepath.Join("data", "misc", "misc", file))
+	data, err := os.ReadFile(filepath.Join("data", "misc", file))
 	if err != nil {
 		t.Fatal(err)
 	}