As an aside, there are also ISO B and C series paper, although I believe that the C standard was withdrawn somewhat recently. The area of B_n is the geometric mean of that of A_n and A_{n-1}, while the area of C_n is the geometric mean of that of A_n and B_n. Without context, C4 paper sounds quite scary, until you realize it's probably related to A4 paper in some way. As to why it isn't just called A3.75, I have no idea.