 The "root directory" of the disk contains the number of the first cluster of each file in that directory; the operating system can then traverse the FAT table, looking up the cluster number of each successive part of the disk file as a "cluster chain" until the end of the file is reached.In much the same way, "sub-directories" are implemented as special files containing the "directory entries" of their respective files.Originally designed as an 8-bit file system, the maximum number of clusters has been significantly increased as disk drives have evolved, and so the number of bits used to identify each cluster has grown.