20
Cardinal	twenty
Ordinal	twentieth
Numeral system	vigesimal
Factorization
Roman numeral	XX
Binary	10100
Hexadecimal	14

Mathematics

It is the total number of digits on the human body, which makes it a possible candidate for a numeral system. Twenty is used as a radix for vigesimal number systems. In the old UK currency system, there were twenty shillings in a pound.

Other fields

In a game of chess both players have 20 first moves from which to choose.

The number 20 is used as an index in measuring visual acuity. 20/20 indicates perfect vision. 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".

Twenty questions is a popular party game.

Twenty is the age of majority in Japanese tradition. Someone who is exactly twenty years old is described as hatachi.

The "Twenty Year Curse" refers to the pattern of the president of the United States elected every twenty years starting with 1840 to die in office. The pattern was broken by Reagan, who survived a would-be assassin's bullet.

Bands with the number twenty in their name include Matchbox 20.

A group of twenty units is often referred to as a score.

A curious survival from the Celtic languages is preserved in folklore from the north of England; this is called the "Lincolnshire Score" or the Shepherd's Score. This is a tally that counts twenty units using the rhyming jingle:

Yan, tan, tethera, pethera, pimp,

sethera, lethera, hovera, dovera, dick,

yanadick, tanadick, tetheradick, petheradick, bumfit,

yanabumfit, tanabumfit, tetherabumfit, petherabumfit, jiggit.

Twenty is:

• The atomic number of calcium.
• The denomination of U.S. dollar with Andrew Jackson's portrait.
• Twenty, a 1997 album by Lynyrd Skynyrd.
• The code for international direct dial phone calls to Egypt.
• The designation of Interstate 20, a freeway that runs from Texas to South Carolina.
• Twenty, a village in Lincolnshire
• The year AD 20.