20 (number)

Bring's curve is a Riemann surface, whose fundamental polygon is a regular hyperbolic icosagon.

When someone is able to see only after an event how things turned out, that person is often said to have had "20/20 hindsight"[7] In many disciplines of developmental psychology, adulthood starts at age 20.

To represent this, the Japanese language has a special word for "20-years-old" that does not follow the rest of their numbering system.

Accordingly, the word 二十歳 is read all at once as "はたち" (hatachi) rather than the expected pronunciation of the three characters as "にじゅうさい" (nijyuusai, which is literally "two," "ten," and the counter for "years old").

20 is the basis for vigesimal number systems, used by several different civilizations in the past (and to this day), including the Maya.