diff --git a/src/GameCult.Caching/CultElementIds.cs b/src/GameCult.Caching/CultElementIds.cs
index 81fada0a..8e10198b 100644
--- a/src/GameCult.Caching/CultElementIds.cs
+++ b/src/GameCult.Caching/CultElementIds.cs
@@ -290,7 +290,7 @@ namespace GameCult.Caching
                         continue;
                     if (shape.Derive == null && !IsRandomId(id!))
                         throw new CultElementIdException(
-                            $"Record {rootPath}: element id '{id}' in the list at {path} is not 12 lowercase hex characters.",
+                            $"Record {rootPath}: element id '{id}' in the list at {path} is not 12 lowercase hex characters. An id you write by hand must be 12 lowercase hex; for readable ids, derive them from a member with [CultElementId(nameof(Member))].",
                             rootPath, path, id);
                     if (!taken.Add(id!))
                         throw Duplicate(id!, path);
diff --git a/src/GameCult.Caching/README.md b/src/GameCult.Caching/README.md
index e9edb63d..6e47742f 100644
--- a/src/GameCult.Caching/README.md
+++ b/src/GameCult.Caching/README.md
@@ -188,6 +188,31 @@ for (var row = 0; row < table.Count; row++)
 cache-owned physical table for hot loops and diagnostics; it is not a second
 domain model callers have to maintain.
 
+## Element ids
+
+A document type that holds a list of objects gives every object type in that list
+(union arms included) a string member marked `[CultElementId]`, next to its
+`[Key]`. The id is what lets a variant override one list element instead of the
+whole list. Lists of strings, numbers and other non-object values need none.
+
+```csharp
+[MessagePackObject]
+public sealed class Line
+{
+    [Key(0)] [CultElementId] public string Id = string.Empty;
+    [Key(1)] public string Text = string.Empty;
+}
+```
+
+Leave `Id` empty and the cache mints a random id when the record is written. An
+id you set yourself must be exactly 12 lowercase hex characters
+(`0123456789ab`); anything else is refused with a `CultElementIdException`.
+
+For readable ids, derive them: `[CultElementId(nameof(Name))]` makes an unset id
+the text of that member (a string or number on the same type, unique within its
+list), and a derived id may be any text. A readable name set by hand on a
+non-derived list id is refused; the same member outside a list is unconstrained.
+
 ## Persistence discipline
 
 Persistence stays manual by default. Mutations mark the cache and attached
diff --git a/tests/GameCult.Caching.Tests/ElementIdEngineTests.cs b/tests/GameCult.Caching.Tests/ElementIdEngineTests.cs
index dae82cee..7ae2ee20 100644
--- a/tests/GameCult.Caching.Tests/ElementIdEngineTests.cs
+++ b/tests/GameCult.Caching.Tests/ElementIdEngineTests.cs
@@ -135,6 +135,13 @@ namespace GameCult.Caching.Tests
             Assert.Throws<CultElementIdException>(() => Assign(grid, "r", deterministic: false));
         }
 
+        [Test]
+        public void TheHexRefusalNamesTheDerivedFormForReadableIds()
+        {
+            var refusal = Assert.Throws<CultElementIdException>(() => Assign(new HHasIds { Items = { new EIded { Id = "root" } } }, "r", deterministic: false))!;
+            Assert.That(refusal.Message, Does.Contain("12 lowercase hex").And.Contain("[CultElementId(nameof(Member))]"));
+        }
+
         [Test]
         public void ARandomIdMustBeTwelveLowercaseHexCharactersAndADerivedIdNeedOnlyBeText()
         {