diff --git a/nse_main.cc b/nse_main.cc
index 701bd02e4..ed062e71e 100644
--- a/nse_main.cc
+++ b/nse_main.cc
@@ -22,7 +22,8 @@
 #define NSE_MAIN "NSE_MAIN" /* the main function */
 #define NSE_TRACEBACK "NSE_TRACEBACK"
 
-/* string keys used in interface with nse_main.lua */
+/* These are indices into the registry, for data shared with nse_main.lua. The
+   definitions here must match those in nse_main.lua. */
 #define NSE_YIELD "NSE_YIELD"
 #define NSE_BASE "NSE_BASE"
 #define NSE_WAITING_TO_RUNNING "NSE_WAITING_TO_RUNNING"
diff --git a/nse_main.lua b/nse_main.lua
index 2ebeefaea..d0270d24f 100644
--- a/nse_main.lua
+++ b/nse_main.lua
@@ -32,6 +32,7 @@
 
 local NAME = "NSE";
 
+-- String keys into the registry (_R), for data shared with nse_main.cc.
 local YIELD = "NSE_YIELD";
 local BASE = "NSE_BASE";
 local WAITING_TO_RUNNING = "NSE_WAITING_TO_RUNNING";