diff --git a/spec/Section 4 -- Introspection.md b/spec/Section 4 -- Introspection.md
index 3aa4e40e0..3054a9f6c 100644
--- a/spec/Section 4 -- Introspection.md	
+++ b/spec/Section 4 -- Introspection.md	
@@ -316,9 +316,10 @@ Fields\:
 **Interface**
 
 Interfaces are an abstract type where there are common fields declared. Any type
-that implements an interface must define all the fields with names and types
-exactly matching. The implementations of this interface are explicitly listed
-out in `possibleTypes`.
+that implements an interface must define all the named fields where each
+implementing field type is equal to or a sub-type of (covariant) the interface
+type. The implementations of this interface are explicitly listed out in
+`possibleTypes`.
 
 Fields\: