diff --git a/spec/abi.dd b/spec/abi.dd
index 32a00ce71c..4562a043ca 100644
--- a/spec/abi.dd
+++ b/spec/abi.dd
@@ -68,9 +68,14 @@ $(H2 $(LNAME2 delegates, Delegates))
         function's stack frame (nested functions).
         )
 
-$(H2 $(LNAME2 structs, Structs))
+$(H2 $(LNAME2 structs, Structs and Unions))
 
-        $(P Conforms to the target's C ABI struct layout.)
+        $(P These conform to the target's C ABI struct layout, except:)
+
+        * An `extern(D)` struct or union with no fields has size 1, not 0.
+          See $(DDSUBLINK spec/struct, struct_layout, Struct Layout).
+        * Unions are $(RELATIVE_LINK2 name_mangling, name-mangled) as if they were structs.
+        * The rest of this document treats unions extrinsically the same as structs.
 
 $(H2 $(LNAME2 classes, Classes))
 
diff --git a/spec/struct.dd b/spec/struct.dd
index c0dcf5c176..efdabc0c5c 100644
--- a/spec/struct.dd
+++ b/spec/struct.dd
@@ -176,7 +176,18 @@ $(H2 $(LNAME2 struct_layout, Struct Layout))
     the first field and the start of the object.
     )
 
-    $(P Structs with no fields of non-zero size (aka $(I Empty Structs)) have a size of one byte.)
+    $(P $(DDSUBLINK spec/attribute, linkage, `extern(D)`) structs with no fields of non-zero
+    size (aka $(I Empty Structs)) have a size of one byte.)
+
+    $(SPEC_RUNNABLE_EXAMPLE_COMPILE
+    ---
+    extern(C) struct C {}
+    struct D {}
+
+    static assert(C.sizeof == 0);
+    static assert(D.sizeof == 1);
+    ---
+    )
 
     $(P Non-static $(RELATIVE_LINK2 nested, function-nested D structs), which access the context of
     their enclosing scope, have an extra field.