Martin uses letters (O, OO, OOO, OM, D) to indicate the size of the guitar and a number (18, 21, 28...) to indicate the materials used to make them.

Martin still makes their traditional quality guitars, the 18s, 21s, 28s, 35s, 42s and 45s, but they have started making more affordable guitars as well. Some of these are very nice guitars, but don`t expect the quality of the traditional models.