# twenty

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).