diff --git a/ident/member.go b/ident/member.go
new file mode 100644
index 0000000..d4b0eb6
--- /dev/null
+++ b/ident/member.go
@@ -0,0 +1,84 @@
+package ident
+
+import (
+	"encoding/base64"
+	"encoding/binary"
+	"unsafe"
+)
+
+// M represents a unique member identifier.
+type M struct {
+	// A per-system value incremented by some unspecified amount every time the
+	// metadata of a member first appears to the backend.
+	Serial uint64
+	// An instant in time, some time after the corresponding member metadata
+	// first appeared to the backend, represented in nanoseconds since
+	// 1970-01-01.
+	Time uint64
+	// Randomly generated value. The implementation must guarantee that the same
+	// value cannot be emitted for a Time value.
+	ID uint64
+
+	// Underlying system.
+	System S
+}
+
+const (
+	// SizeMember is the size of the binary representation of [M].
+	SizeMember = SizeSystem * 2
+	// EncodedSizeMember is the size of the string representation of [M].
+	EncodedSizeMember = SizeMember / 3 * 4
+)
+
+// Encode appends the canonical string representation of mid to dst and returns
+// the extended buffer.
+func (mid *M) Encode(dst []byte) []byte {
+	var buf [SizeMember - SizeSystem]byte
+
+	p := buf[:]
+	binary.LittleEndian.PutUint64(p, mid.Serial)
+	p = p[8:]
+	binary.LittleEndian.PutUint64(p, mid.Time)
+	p = p[8:]
+	binary.LittleEndian.PutUint64(p, mid.ID)
+	dst = base64.URLEncoding.AppendEncode(dst, buf[:])
+
+	return mid.System.Encode(dst)
+}
+
+// String returns the canonical string representation of mid.
+func (mid *M) String() string {
+	s := mid.Encode(nil)
+	return unsafe.String(unsafe.SliceData(s), len(s))
+}
+
+// MarshalText returns the result of Encode.
+func (mid *M) MarshalText() (data []byte, err error) {
+	return mid.Encode(nil), nil
+}
+
+// UnmarshalText strictly decodes data into mid.
+func (mid *M) UnmarshalText(data []byte) error {
+	if len(data) != EncodedSizeMember {
+		return &UnexpectedSizeError{data, EncodedSizeMember}
+	}
+
+	var buf [SizeMember - SizeSystem]byte
+	if n, err := base64.URLEncoding.Decode(
+		buf[:],
+		data[:EncodedSizeMember-EncodedSizeSystem],
+	); err != nil {
+		return err
+	} else if n != SizeMember-SizeSystem {
+		return ErrNewline
+	}
+
+	p := buf[:]
+	mid.Serial = binary.LittleEndian.Uint64(p)
+	p = p[8:]
+	mid.Time = binary.LittleEndian.Uint64(p)
+	p = p[8:]
+	mid.ID = binary.LittleEndian.Uint64(p)
+
+	return mid.System.UnmarshalText(data[EncodedSizeMember-EncodedSizeSystem:])
+}
diff --git a/ident/member_test.go b/ident/member_test.go
new file mode 100644
index 0000000..a50ad98
--- /dev/null
+++ b/ident/member_test.go
@@ -0,0 +1,84 @@
+package ident_test
+
+import (
+	"encoding/base64"
+	"strings"
+	"testing"
+	"time"
+
+	"git.gensokyo.uk/cofront/cof-spec/ident"
+)
+
+func TestM(t *testing.T) {
+	t.Parallel()
+
+	rTestCases[ident.M, *ident.M]{
+		{"short", ident.M{}, nil, &ident.UnexpectedSizeError{
+			Data: nil,
+			Want: ident.EncodedSizeMember,
+		}},
+
+		{"malformed", ident.M{}, []byte{
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xe1, 0xde, 0xad,
+		}, base64.CorruptInputError(0)},
+
+		{"newline", ident.M{}, []byte(
+			"AAAA" + strings.Repeat("\n", ident.EncodedSizeMember-4),
+		), ident.ErrNewline},
+
+		{"valid", ident.M{
+			Serial: 0xfdfdfdfdfdfdfdfd,
+
+			Time: uint64(time.Date(
+				0xfd, 7, 15,
+				23, 59, 59, 0xcab,
+				time.UTC,
+			).UnixNano()),
+
+			ID: 0x2e736e64,
+
+			System: ident.S{
+				Site: ident.TrivialSite,
+				Host: ident.TrivialHost,
+
+				Time: uint64(time.Date(
+					0xfd, 7, 15,
+					23, 59, 59, 0xcafe,
+					time.UTC,
+				).UnixNano()),
+
+				ID: 0xfee1dead0badf00d,
+			},
+		}, base64.URLEncoding.AppendEncode(nil, []byte{
+			/* member */
+
+			/* serial: */ 0xfd, 0xfd, 0xfd, 0xfd, 0xfd, 0xfd, 0xfd, 0xfd,
+			/* time:   */ 0xab, 0x42, 0x42, 0xce, 0xf1, 0x92, 0x4a, 0x10,
+			/* id:     */ 0x64, 0x6e, 0x73, 0x2e, 0, 0, 0, 0,
+
+			/* system */
+
+			/* site: */ 0xfe, 0xca, 0xed, 0xfe,
+			/* host: */ 0xbe, 0xba, 0xfe, 0xca,
+
+			/* time: */ 0xfe, 0, 0x43, 0xce, 0xf1, 0x92, 0x4a, 0x10,
+			/* id:   */ 0xd, 0xf0, 0xad, 0xb, 0xad, 0xde, 0xe1, 0xfe,
+		}), nil},
+	}.run(t)
+}