An S(2,3,"n") is called a Steiner triple system, and its blocks are called triples. It is common to see the abbreviation STS("n") for a Steiner triple system of order "n". The total number of pairs is "n(n-1)/2", of which three appear in a triple, and so the total number of triples is "n"("n"−1)/6. This shows that "n" must be of the form "6k+1" or "6k + 3" for some "k". The fact that this condition on "n" is sufficient for the existence of an S(2,3,"n") was proved by Raj Chandra Bose and T. Skolem. The projective plane of order 2 (the Fano plane) is an STS(7) and the affine plane of order 3 is an STS(9). Up to isomorphism, the STS(7) and STS(9) are unique, there are two STS(13)s, 80 STS(15)s, and 11,084,874,829 STS(19)s.