The Maya numerals are an example of a base-20 numeral system.

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A vigesimal (/vɪˈdʒɛsɪməl/ vij-ESS-im-əl) or base-20 (base-score) numeral system is based on twenty (in the same way in which the decimal numeral system is based on ten). Vigesimal is derived from the Latin adjective vicesimus, meaning 'twentieth'.

In a vigesimal place system, twenty individual numerals (or digit symbols) are used, ten more than in the decimal system. One modern method of finding the extra needed symbols is to write ten as the letter A, or A₂₀ , where the ₂₀ means base 20, to write nineteen as J₂₀, and the numbers between with the corresponding letters of the alphabet. This is similar to the common computer-science practice of writing hexadecimal numerals over 9 with the letters "A–F". Another less common method skips over the letter "I", in order to avoid confusion between I₂₀ as eighteen and one, so that the number eighteen is written as J₂₀, and nineteen is written as K₂₀. The number twenty is written as 10₂₀.

Vigesimal multiplication table

1	2	3	4	5	6	7	8	9	A	B	C	D	E	F	G	H	I	J	10
2	4	6	8	A	C	E	G	I	10	12	14	16	18	1A	1C	1E	1G	1I	20
3	6	9	C	F	I	11	14	17	1A	1D	1G	1J	22	25	28	2B	2E	2H	30
4	8	C	G	10	14	18	1C	1G	20	24	28	2C	2G	30	34	38	3C	3G	40
5	A	F	10	15	1A	1F	20	25	2A	2F	30	35	3A	3F	40	45	4A	4F	50
6	C	I	14	1A	1G	22	28	2E	30	36	3C	3I	44	4A	4G	52	58	5E	60
7	E	11	18	1F	22	29	2G	33	3A	3H	44	4B	4I	55	5C	5J	66	6D	70
8	G	14	1C	20	28	2G	34	3C	40	48	4G	54	5C	60	68	6G	74	7C	80
9	I	17	1G	25	2E	33	3C	41	4A	4J	58	5H	66	6F	74	7D	82	8B	90
A	10	1A	20	2A	30	3A	40	4A	50	5A	60	6A	70	7A	80	8A	90	9A	A0
B	12	1D	24	2F	36	3H	48	4J	5A	61	6C	73	7E	85	8G	97	9I	A9	B0
C	14	1G	28	30	3C	44	4G	58	60	6C	74	7G	88	90	9C	A4	AG	B8	C0
D	16	1J	2C	35	3I	4B	54	5H	6A	73	7G	89	92	9F	A8	B1	BE	C7	D0
E	18	22	2G	3A	44	4I	5C	66	70	7E	88	92	9G	AA	B4	BI	CC	D6	E0
F	1A	25	30	3F	4A	55	60	6F	7A	85	90	9F	AA	B5	C0	CF	DA	E5	F0
G	1C	28	34	40	4G	5C	68	74	80	8G	9C	A8	B4	C0	CG	DC	E8	F4	G0
H	1E	2B	38	45	52	5J	6G	7D	8A	97	A4	B1	BI	CF	DC	E9	F6	G3	H0
I	1G	2E	3C	4A	58	66	74	82	90	9I	AG	BE	CC	DA	E8	F6	G4	H2	I0
J	1I	2H	3G	4F	5E	6D	7C	8B	9A	A9	B8	C7	D6	E5	F4	G3	H2	I1	J0
10	20	30	40	50	60	70	80	90	A0	B0	C0	D0	E0	F0	G0	H0	I0	J0	100

Decimal	Vigesimal
0	0
1	1
2	2
3	3
4	4
5	5
6	6
7	7
8	8
9	9
10	A
11	B
12	C
13	D
14	E
15	F
16	G
17	H
18	I	J
19	J	K

According to this notation:

20₂₀ is equivalent to forty in decimal = (2 × 20¹) + (0 × 20⁰)

D0₂₀ is equivalent to two hundred and sixty in decimal = (13 × 20¹) + (0 × 20⁰)

100₂₀ is equivalent to four hundred in decimal = (1 × 20²) + (0 × 20¹) + (0 × 20⁰).

In the rest of this article below, numbers are expressed in decimal notation, unless specified otherwise. For example, 10 means ten, 20 means twenty. Numbers in vigesimal notation use the convention that I means eighteen and J means nineteen.

2, 5

⁠1/10⁠

The prime factorization of twenty is 2² × 5, so it is not a perfect power. However, its squarefree part, 5, is congruent to 1 (mod 4). Thus, according to Artin's conjecture on primitive roots, vigesimal has infinitely many cyclic primes, but the fraction of primes that are cyclic is not necessarily ~37.395%. An UnrealScript program that computes the lengths of recurring periods of various fractions in a given set of bases found that, of the first 15,456 primes, ~39.344% are cyclic in vigesimal.

Algebraic irrational number	In decimal	In vigesimal
√2 (the length of the diagonal of a unit square)	1.41421356237309...	1.85DE37JGF09H6...
√3 (the length of the diagonal of a unit cube)	1.73205080756887...	1.ECG82BDDF5617...
√5 (the length of the diagonal of a 1 × 2 rectangle)	2.2360679774997...	2.4E8AHAB3JHGIB...
φ (phi, the golden ratio = ⁠1+√5/2⁠)	1.6180339887498...	1.C7458F5BJII95...
Transcendental irrational number	In decimal	In vigesimal
π (pi, the ratio of circumference to diameter)	3.14159265358979...	3.2GCEG9GBHJ9D2...
e (the base of the natural logarithm)	2.7182818284590452...	2.E7651H08B0C95...
γ (the limiting difference between the harmonic series and the natural logarithm)	0.5772156649015328606...	0.BAHEA2B19BDIBI...

In several European languages like French and Danish, 20 is used as a base, at least with respect to the linguistic structure of the names of certain numbers (though a thoroughgoing consistent vigesimal system, based on the powers 20, 400, 8000 etc., is not generally used).

Many cultures that use a vigesimal system count in fives to twenty, then count twenties similarly. Such a system is referred to as quinary-vigesimal by linguists. Examples include Greenlandic, Iñupiaq, Kaktovik, Maya, Nunivak Cupʼig, and Yupʼik numerals.^[1][2][3]

Vigesimal systems are common in Africa, for example in Yoruba.^[4] While the Yoruba number system may be regarded as a vigesimal system, it is complex.^{[further explanation needed]}

• Twenty is a base in the Maya and Aztec number systems. The Maya use the following names for the powers of twenty: kal (20), bak (20² = 400), pic (20³ = 8,000), calab (20⁴ = 160,000), kinchil (20⁵ = 3,200,000) and alau (20⁶ = 64,000,000). See Maya numerals and Maya calendar, Nahuatl language.
• The Inuit-Yupik-Unangax languages have base-20 number systems. In 1994, Inuit students in Kaktovik, Alaska, came up with the base-20 Kaktovik numerals to better represent their language. Before this invention led to a revival, the Inuit numerals had been falling out of use.^[5] The Kaktovik numerals are:

0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19