diff --git a/CHANGES b/CHANGES
index 3aad821..b4495c9 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,6 @@
+0.10:
+  add comment to stralloc.h that explains the structure (Markus Brischke)
+
 0.9:
   fmt_str did not check for out==NULL!  Thanks, Uwe Ohse.
   Updated to buffer to fix read buffers.  Thanks, David Lichteblau.
@@ -6,6 +9,7 @@
   add mmap man pages.
   update and add socket man pages.
   don't include str.h from fmt.h
+  document error signalling for the mmap functions.
 
 0.8:
   BSD compatibility.
diff --git a/stralloc.h b/stralloc.h
index 1999fa4..a9a6790 100644
--- a/stralloc.h
+++ b/stralloc.h
@@ -3,6 +3,12 @@
 
 #include <sys/cdefs.h>
 
+/* stralloc is the internal data structure all functions are working on.
+ * s is the string.
+ * len is the used length of the string.
+ * a is the allocated length of the string.
+ */
+
 typedef struct stralloc {
   char* s;
   unsigned int len;