diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/const.h b/libbuild2-autoconf/libbuild2/autoconf/checks/const.h
index abff54f..c86d055 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/const.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/const.h
@@ -1,2 +1,8 @@
 // const
+
+/* This check is unusual in that for a positive answer (the compiler supports
+   const) we undefine const to allow the compiler to recognize it as a keyword
+   and for a negative answer we define it to nothing so that const is
+   effectively removed from the user code. */
+
 #undef const
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/inline.h b/libbuild2-autoconf/libbuild2/autoconf/checks/inline.h
index d0b9ec9..804fa91 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/inline.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/inline.h
@@ -1,2 +1,8 @@
 // inline
+
+/* This check is unusual in that for a positive answer (the compiler supports
+   inline) we undefine inline to allow the compiler to recognize it as a
+   keyword and for a negative answer we define it to nothing so that inline is
+   effectively removed from the user code. */
+
 #undef inline
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/volatile.h b/libbuild2-autoconf/libbuild2/autoconf/checks/volatile.h
index f3b8f67..ead70a3 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/volatile.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/volatile.h
@@ -1,2 +1,8 @@
 // volatile
+
+/* This check is unusual in that for a positive answer (the compiler supports
+   volatile) we undefine volatile to allow the compiler to recognize it as a
+   keyword and for a negative answer we define it to nothing so that volatile
+   is effectively removed from the user code. */
+
 #undef volatile