It's either a short cage, SS, or a long cage, GS. There are no Shimano medium cage road derailleurs. The SS is sold with double cranksets, the GS is sold with triple cranksets and may be used with double cranksets. Shimano does make some medium cage mountain derailleurs, but the 105 is a road RD. Take a look at the examples shown at http://www.shimano.com.