Smiths Cat,
from "cruise-charter.net"
Let us take a sloop - its working sails are a mainsail and a working jib. The jib covers most of the fore-triangle (everything between forestay, deck and mast). The main sail plus the jib are the boat's primary source of speed. However, to get the most of it, to explore its limit to their extremes, you need one other sail called a Genoa.

The Genoa, or as they call it Genny, is a simple jib, whose clew reaches behind the mast (or also called abaft). Its size is directly related to the fore-triangle, and in fact it is calculated as a percentage of it. There are 4 main types of Genoa - the smallest is called Lapper (it is 110% of the fore-triangle, only 10% bigger than it), then we have Genoa 2 (135%), Genoa 3 (150%) and the Drifter (160% or even 165%). The name of the sail came from a regatta in 1927 for 6-m yacht; which was crushingly won by a crew which used a Genoa - extra sail attached to the jib; there were no rules banning this, so they won by this innovation and thus it took its name. The Genoa is not only for speed races in regattas, it has irreplaceable contribution when there is very light wind or still. It uses the space optimally, not even one cm remains open and the drive created is really considerable. The only negative side is that it blocks the forward view of the helmsman; and he/she better have a separate person from the crew to handle the Genoa. The sheets of the Genoa (ropes) are always run outside the shrouds.