This highly compact form makes the individual chromosomes visible, and they form the classic four arm structure, a pair of sister chromatids attached to each other at the centromere. The shorter arms are called "p arms" (from the French "petit", small) and the longer arms are called "q arms" ("q" follows "p" in the Latin alphabet; q-g "grande"; alternatively it is sometimes said q is short for "queue" meaning tail in French). This is the only natural context in which individual chromosomes are visible with an optical microscope.