diff --git a/README.md b/README.md
index 209fd1d..5782f08 100644
--- a/README.md
+++ b/README.md
@@ -95,6 +95,7 @@ The package caches each type's definition, so `Definition` must always return th
 | `JSONFormat` | `FormatFull` | Form for `MarshalJSON` and `UnmarshalJSON` |
 | `Validate` | none | Checks the bare value; use `ValidateUUID`, `ValidateInteger` or your own function. `ValidateUUID` accepts only the lowercase hyphenated form |
 | `Generate` | none | Makes a bare value for `Generate`; use `GenerateUUIDv7`, `GenerateUUIDv4` or your own function |
+| `Aliases` | none | Earlier object types that parsing still accepts. Encoders always emit `ObjectType`, so a prefix can be renamed without breaking stored or sent values |
 
 ### Check definitions in a test
 
diff --git a/alias_test.go b/alias_test.go
new file mode 100644
index 0000000..d049f7d
--- /dev/null
+++ b/alias_test.go
@@ -0,0 +1,97 @@
+package trogonobjectid_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/TrogonStack/trogonobjectid"
+)
+
+type paymentMethodType struct{}
+
+func (paymentMethodType) Definition() trogonobjectid.Definition {
+	return trogonobjectid.Definition{ObjectType: "pm", Aliases: []trogonobjectid.ObjectType{"card", "src"}}
+}
+
+type selfAliasType struct{}
+
+func (selfAliasType) Definition() trogonobjectid.Definition {
+	return trogonobjectid.Definition{ObjectType: "pm", Aliases: []trogonobjectid.ObjectType{"pm"}}
+}
+
+type badAliasType struct{}
+
+func (badAliasType) Definition() trogonobjectid.Definition {
+	return trogonobjectid.Definition{ObjectType: "pm", Aliases: []trogonobjectid.ObjectType{"Card"}}
+}
+
+type paymentUnion struct{}
+
+func (paymentUnion) Types() []trogonobjectid.MemberType {
+	return []trogonobjectid.MemberType{
+		trogonobjectid.MemberOf[paymentMethodType](),
+		trogonobjectid.MemberOf[userType](),
+	}
+}
+
+type clashUnion struct{}
+
+func (clashUnion) Types() []trogonobjectid.MemberType {
+	return []trogonobjectid.MemberType{
+		trogonobjectid.MemberOf[paymentMethodType](),
+		trogonobjectid.MemberOf[cardType](),
+	}
+}
+
+type cardType struct{}
+
+func (cardType) Definition() trogonobjectid.Definition {
+	return trogonobjectid.Definition{ObjectType: "card"}
+}
+
+func TestAliases(t *testing.T) {
+	want := trogonobjectid.MustNew[paymentMethodType]("abc")
+
+	for _, input := range []string{"pm_abc", "card_abc", "src_abc"} {
+		t.Run("Parse accepts "+input, func(t *testing.T) {
+			got, err := trogonobjectid.Parse[paymentMethodType](input)
+			require.NoError(t, err)
+			assert.Equal(t, want, got)
+			assert.Equal(t, "pm_abc", got.String())
+		})
+	}
+
+	t.Run("full storage reads an aliased value", func(t *testing.T) {
+		var got trogonobjectid.ID[paymentMethodType]
+		require.NoError(t, got.Scan("card_abc"))
+		assert.Equal(t, want, got)
+	})
+
+	t.Run("an unknown prefix still fails", func(t *testing.T) {
+		_, err := trogonobjectid.Parse[paymentMethodType]("user_abc")
+		require.ErrorIs(t, err, trogonobjectid.ErrInvalidFormat)
+	})
+
+	t.Run("a union matches an alias", func(t *testing.T) {
+		u, err := trogonobjectid.ParseUnion[paymentUnion]("card_abc")
+		require.NoError(t, err)
+		got, ok := trogonobjectid.UnionAs[paymentMethodType](u)
+		require.True(t, ok)
+		assert.Equal(t, want, got)
+		assert.Equal(t, "pm_abc", u.String())
+	})
+
+	t.Run("Check rejects an alias equal to the object type", func(t *testing.T) {
+		require.ErrorIs(t, trogonobjectid.Check[selfAliasType](), trogonobjectid.ErrInvalidDefinition)
+	})
+
+	t.Run("Check rejects an invalid alias", func(t *testing.T) {
+		require.ErrorIs(t, trogonobjectid.Check[badAliasType](), trogonobjectid.ErrInvalidDefinition)
+	})
+
+	t.Run("CheckUnion rejects an alias that is another member's prefix", func(t *testing.T) {
+		require.ErrorIs(t, trogonobjectid.CheckUnion[clashUnion](), trogonobjectid.ErrInvalidDefinition)
+	})
+}
diff --git a/objectid.go b/objectid.go
index cd92e06..43608cf 100644
--- a/objectid.go
+++ b/objectid.go
@@ -70,6 +70,9 @@ type Definition struct {
 	Validate func(id string) error
 	// Generate returns a new bare value for Generate. Nil makes Generate fail.
 	Generate func() (string, error)
+	// Aliases are earlier object types that parsing still accepts. Every encoder emits
+	// ObjectType, so stored and sent values move to the new prefix as they are rewritten.
+	Aliases []ObjectType
 }
 
 // Definer is implemented by the marker type that names an object ID type.
@@ -99,7 +102,8 @@ type ID[T Definer] struct {
 
 type spec struct {
 	Definition
-	prefix string
+	// prefixes holds the ObjectType's prefix first, then each alias's.
+	prefixes []Prefix
 	// objectTypes is the one-element list that every ValidationError carries.
 	objectTypes []ObjectType
 }
@@ -121,9 +125,21 @@ func newSpec[T Definer]() (spec, error) {
 	if def.Separator == "" {
 		def.Separator = DefaultSeparator
 	}
+	prefixes := []Prefix{Prefix(string(def.ObjectType) + string(def.Separator))}
+	seen := map[ObjectType]bool{def.ObjectType: true}
+	for _, alias := range def.Aliases {
+		if err := alias.validate(); err != nil {
+			return spec{}, fmt.Errorf("%w: %T alias: %w", ErrInvalidDefinition, marker, err)
+		}
+		if seen[alias] {
+			return spec{}, fmt.Errorf("%w: %T repeats object type %q in its aliases", ErrInvalidDefinition, marker, alias)
+		}
+		seen[alias] = true
+		prefixes = append(prefixes, Prefix(string(alias)+string(def.Separator)))
+	}
 	return spec{
 		Definition:  def,
-		prefix:      string(def.ObjectType) + string(def.Separator),
+		prefixes:    prefixes,
 		objectTypes: []ObjectType{def.ObjectType},
 	}, nil
 }
@@ -141,17 +157,18 @@ func (s *spec) check(id string) error {
 }
 
 func (s *spec) parse(full string) (string, error) {
-	bare, ok := strings.CutPrefix(full, s.prefix)
-	if !ok {
-		return "", &ValidationError{ObjectTypes: s.objectTypes, Value: full, Err: &WrongPrefixError{Want: []Prefix{Prefix(s.prefix)}}}
-	}
-	if bare == "" {
-		return "", &ValidationError{ObjectTypes: s.objectTypes, Value: full, Err: ErrInvalidFormat}
-	}
-	if err := s.check(bare); err != nil {
-		return "", err
+	for _, p := range s.prefixes {
+		if bare, ok := strings.CutPrefix(full, string(p)); ok {
+			if bare == "" {
+				return "", &ValidationError{ObjectTypes: s.objectTypes, Value: full, Err: ErrInvalidFormat}
+			}
+			if err := s.check(bare); err != nil {
+				return "", err
+			}
+			return bare, nil
+		}
 	}
-	return bare, nil
+	return "", &ValidationError{ObjectTypes: s.objectTypes, Value: full, Err: &WrongPrefixError{Want: s.prefixes}}
 }
 
 func (s *spec) decode(format Format, value string) (string, error) {
@@ -168,7 +185,7 @@ func (s *spec) encode(format Format, id string) string {
 	if format == FormatDropPrefix {
 		return id
 	}
-	return s.prefix + id
+	return string(s.prefixes[0]) + id
 }
 
 // New returns an ID for the bare value id, after validation.
@@ -283,7 +300,7 @@ func Check[T Definer]() error {
 
 // PrefixOf returns T's object type and separator, for example "user_". It panics if
 // T's Definition is invalid.
-func PrefixOf[T Definer]() Prefix { return Prefix(mustSpec[T]().prefix) }
+func PrefixOf[T Definer]() Prefix { return mustSpec[T]().prefixes[0] }
 
 // IsZero reports whether id is the zero (absent) ID.
 func (id ID[T]) IsZero() bool { return id.id == "" }
@@ -302,7 +319,7 @@ func (id ID[T]) String() string {
 	if id.IsZero() {
 		return ""
 	}
-	return mustSpec[T]().prefix + id.id
+	return string(mustSpec[T]().prefixes[0]) + id.id
 }
 
 // Storage returns the value in the type's StorageFormat, or "" for the zero ID.
diff --git a/union.go b/union.go
index e85d516..5126c0f 100644
--- a/union.go
+++ b/union.go
@@ -21,7 +21,7 @@ type Member interface {
 // MemberType describes one ID type that a Union accepts. Make it with MemberOf.
 type MemberType struct {
 	objectType ObjectType
-	prefix     string
+	prefixes   []Prefix
 	err        error
 	parse      func(s string) (Member, error)
 	holds      func(m Member) bool
@@ -32,7 +32,7 @@ func MemberOf[T Definer]() MemberType {
 	s, err := specFor[T]()
 	return MemberType{
 		objectType: s.ObjectType,
-		prefix:     s.prefix,
+		prefixes:   s.prefixes,
 		err:        err,
 		parse:      func(str string) (Member, error) { return Parse[T](str) },
 		holds:      func(m Member) bool { _, ok := m.(ID[T]); return ok },
@@ -67,6 +67,13 @@ type Union[U UnionDefiner] struct {
 type unionSpec struct {
 	objectTypes []ObjectType
 	members     []MemberType
+	// entries pairs every member prefix, aliases included, with its member, longest first.
+	entries []unionEntry
+}
+
+type unionEntry struct {
+	prefix Prefix
+	member MemberType
 }
 
 var unionSpecs sync.Map
@@ -83,22 +90,25 @@ func newUnionSpec[U UnionDefiner]() (unionSpec, error) {
 	}
 
 	objectTypes := make([]ObjectType, 0, len(members))
-	seen := make(map[string]bool, len(members))
+	seen := make(map[Prefix]bool, len(members))
+	var entries []unionEntry
 	for _, m := range members {
 		if m.err != nil {
 			return unionSpec{}, fmt.Errorf("%T: %w", marker, m.err)
 		}
-		if seen[m.prefix] {
-			return unionSpec{}, fmt.Errorf("%w: %T has duplicate prefix %q", ErrInvalidDefinition, marker, m.prefix)
+		for _, p := range m.prefixes {
+			if seen[p] {
+				return unionSpec{}, fmt.Errorf("%w: %T has duplicate prefix %q", ErrInvalidDefinition, marker, p)
+			}
+			seen[p] = true
+			entries = append(entries, unionEntry{prefix: p, member: m})
 		}
-		seen[m.prefix] = true
 		objectTypes = append(objectTypes, m.objectType)
 	}
 
 	// Longest prefix first, so "acme_admin_" wins over "acme_" for "acme_admin_xyz".
-	sorted := append([]MemberType(nil), members...)
-	sort.SliceStable(sorted, func(i, j int) bool { return len(sorted[i].prefix) > len(sorted[j].prefix) })
-	return unionSpec{objectTypes: objectTypes, members: sorted}, nil
+	sort.SliceStable(entries, func(i, j int) bool { return len(entries[i].prefix) > len(entries[j].prefix) })
+	return unionSpec{objectTypes: objectTypes, members: members, entries: entries}, nil
 }
 
 // CheckUnion reports whether U's member list is usable: at least one member, valid
@@ -141,18 +151,18 @@ func ParseUnion[U UnionDefiner](str string) (Union[U], error) {
 	if err != nil {
 		return Union[U]{}, err
 	}
-	for _, mt := range s.members {
-		if strings.HasPrefix(str, mt.prefix) {
-			m, err := mt.parse(str)
+	for _, e := range s.entries {
+		if strings.HasPrefix(str, string(e.prefix)) {
+			m, err := e.member.parse(str)
 			if err != nil {
 				return Union[U]{}, err
 			}
 			return Union[U]{member: m}, nil
 		}
 	}
-	prefixes := make([]Prefix, len(s.members))
-	for i, mt := range s.members {
-		prefixes[i] = Prefix(mt.prefix)
+	prefixes := make([]Prefix, len(s.entries))
+	for i, e := range s.entries {
+		prefixes[i] = e.prefix
 	}
 	return Union[U]{}, &ValidationError{ObjectTypes: s.objectTypes, Value: str, Err: &WrongPrefixError{Want: prefixes}}
 }