The term kaon refers to any one of four mesons with nonzero strangeness. The positively charged K+ is composed of an up quark and an anti-strange quark. Its antiparticle is the negatively charged K– , which is composed of an anti-up quark and a strange quark. The two neutral kaons, and are made of down, anti-down, strange, and anti-strange quarks. CP violation was first observed in the neutral kaon system.