In 1976, Shugart Associates introduced the -inch FDD. By 1978, there were more than 10 manufacturers producing such FDDs. There were competing floppy disk formats, with hard- and soft-sector versions and encoding schemes such as differential Manchester encoding (DM), modified frequency modulation (MFM), M2FM and group coded recording (GCR). The -inch format displaced the 8-inch one for most uses, and the hard-sectored disk format disappeared. The most common capacity of the -inch format in DOS-based PCs was 360 KB, for the Double-Sided Double-Density (DSDD) format using MFM encoding. In 1984, IBM introduced with its PC-AT model the 1.2 MB dual-sided -inch floppy disk, but it never became very popular. IBM started using the 720 KB double density -inch microfloppy disk on its Convertible laptop computer in 1986 and the 1.44 MB high-density version with the IBM Personal System/2 (PS/2) line in 1987. These disk drives could be added to older PC models. In 1988, IBM introduced a drive for 2.88 MB Double-Sided Extended-Density (DSED) diskettes in its top-of-the-line PS/2 models, but this was a commercial failure.