diff --git a/src/obidmscolumn.h b/src/obidmscolumn.h
index 96f28d8..6a8b1db 100644
--- a/src/obidmscolumn.h
+++ b/src/obidmscolumn.h
@@ -66,7 +66,7 @@ typedef struct OBIDMS_column_header {
 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 */
 	index_t				line_count;							    			/**< Number of lines of data allocated.
 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 */
-	index_t				lines_used;							    			/**< Number of lines of data used.
+	index_t				lines_used;							    			/**< Number of lines of data used (the highest index where data has been entered + 1).
 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 */
 	index_t				nb_elements_per_line;   			   				/**< Number of elements per line.
 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 	 */