diff --git a/docs/mainframe-design.md b/docs/mainframe-design.md
index d0df433..dc9b567 100644
--- a/docs/mainframe-design.md
+++ b/docs/mainframe-design.md
@@ -2,10 +2,10 @@
 
 The components of a Mark I are:
 
-- an CG 101 Central Processing Unit
-- an CG 102 Core Memory Unit
-- an CG 103 Print-Key-Punch
-- an CG 104 Matrix Display
+- a CG 101 Central Processing Unit
+- a CG 102 Core Memory Unit
+- a CG 103 Print-Key-Punch
+- a CG 104 Matrix Display
 
 ### Console