A letter of the Archaic Greek Alphabet, the koppa had for the most part dropped out of use by the time the alphabet began to be standardized in the Fourth Century BC. Nevertheless, the koppa remained a part of the Greek numeric system (its persistence as a numeral is one reason Greek numbers are so confounding).
The koppa represents a "q" (or hard "k") sound. Its traditional placement (like Q) is between π, pi (p) and ρ, rho (r). It looks like the sign for female without the horizontal crossbar. That is to say, it is a circle placed above the midline, with a stem decending from 270° to the baseline.