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diff --git a/README.md b/README.md
index 239e69a2..a17a3458 100644
--- a/README.md
+++ b/README.md
@@ -46,3 +46,25 @@ database/API (but is useful for paper lookups).
     - [ ] Authentication (eg, accounts, OAuth2, JWT)
     - [ ] Authorization (aka, roles)
     - [ ] bot vs. editor
+
+## Identifiers
+
+Fatcat entity identifiers are in "boring" 
+
+Python helpers for conversion:
+
+    import base64
+    import uuid
+
+    def fcid2uuid(s):
+        s = s.split('_')[-1].upper().encode('utf-8')
+        assert len(s) == 26
+        raw = base64.b32decode(s + b"======")
+        return str(uuid.UUID(bytes=raw)).lower()
+
+    def uuid2fcid(s):
+        raw = uuid.UUID(s).bytes
+        return base64.b32encode(raw)[:26].lower().decode('utf-8')
+
+    test_uuid = '00000000-0000-0000-3333-000000000001'
+    assert test_uuid == fcid2uuid(uuid2fcid(test_uuid))