diff --git a/C#-and-.NET-docs-supplement.md b/C#-and-.NET-docs-supplement.md
index fc5cca3..7b5b78b 100644
--- a/C#-and-.NET-docs-supplement.md
+++ b/C#-and-.NET-docs-supplement.md
@@ -3,7 +3,7 @@ To save us repeating our complaints about the lack of proper documentation under
 
 ## Collection type names
 
-`IReadOnly{Collection,Dictionary,List,Set}` are for getting read-only views of the collections that implement them. They do not mean the collection is immutable (there are separate classes for that).
+`IReadOnly{Collection,Dictionary,List,Set}` are for getting read-only views of the collections that implement them. They do not mean the collection is immutable (there are [separate classes](https://learn.microsoft.com/en-us/dotnet/api/system.collections.immutable?view=net-6.0) for that).
 
 > Kotlin got this right by calling its interfaces e.g. `List`/`MutableList` instead of `IReadOnlyList`/`IList`. (And it also fixed the inheritance hierarchy.)