Many early cultures, including some in Europe, the Mayans of Central America, and the Ainu, the indigenous people of the Japanese islands, used a base of twenty for counting. The base 20 system was retained until about 1970 in the British monetary system, in which there were 20 shillings to the pound. A group of 20 is often called a score. Two of the five Platonic solids involve 20: the icosahedron has 20 triangular faces and the dodecahedron has 20 vertices. Twenty is a Niven number, a semiperfect number (see perfect number), and a practical number. It is also a tetratrahedral number – the sum of consecutive triangular numbers (1 + 3 + 6 + 10).