diff --git a/lib/ext2fs/ext2_fs.h b/lib/ext2fs/ext2_fs.h
index 6c3fb156..8b85d6c5 100644
--- a/lib/ext2fs/ext2_fs.h
+++ b/lib/ext2fs/ext2_fs.h
@@ -149,6 +149,9 @@ struct ext2_group_desc
 	__u16	bg_checksum;		/* crc16(s_uuid+grouo_num+group_desc)*/
 };
 
+/*
+ * Structure of a blocks group descriptor
+ */
 struct ext4_group_desc
 {
 	__u32	bg_block_bitmap;	/* Blocks bitmap block */
@@ -157,17 +160,17 @@ struct ext4_group_desc
 	__u16	bg_free_blocks_count;	/* Free blocks count */
 	__u16	bg_free_inodes_count;	/* Free inodes count */
 	__u16	bg_used_dirs_count;	/* Directories count */
-	__u16	bg_flags;
-	__u32	bg_reserved[2];
+	__u16	bg_flags;		/* EXT4_BG_flags (INODE_UNINIT, etc) */
+	__u32	bg_reserved[2];		/* Likely block/inode bitmap checksum */
 	__u16	bg_itable_unused;	/* Unused inodes count */
-	__u16	bg_checksum;		/* crc16(s_uuid+grouo_num+group_desc)*/
+	__u16	bg_checksum;		/* crc16(sb_uuid+group+desc) */
 	__u32	bg_block_bitmap_hi;	/* Blocks bitmap block MSB */
 	__u32	bg_inode_bitmap_hi;	/* Inodes bitmap block MSB */
 	__u32	bg_inode_table_hi;	/* Inodes table block MSB */
 	__u16	bg_free_blocks_count_hi;/* Free blocks count MSB */
 	__u16	bg_free_inodes_count_hi;/* Free inodes count MSB */
 	__u16	bg_used_dirs_count_hi;	/* Directories count MSB */
-	__u16   bg_pad;
+	__u16	bg_itable_unused_hi;	/* Unused inodes count MSB */
 	__u32	bg_reserved2[3];
 };