十二进制是数学中一种以12为底数的记数系统，通常使用数字0~9以及字母A、B（或X、E）来表示。其中，A（或X）即数字10，B（或E）即数字11。美国速记发明人艾萨克·皮特曼还曾创造过一种标记法，使用翻转的2和3来表示10和11。十二进制中的10代表十进制的12，也称为一打。同样的，十二进制的100代表十进制的144（=12），也称为一篓；十二进制的1000代表十进制的1728（=12），也称为一大篓；而十二进制的0.1则代表十进制的。

12作为一个高合成数，2、3、4、6都是它的因子。正因为如此，十二进制比十进制在有些情况下更易于使用（除了1和10本身，10只有2、5是它的因子）。另外，由于它的因子2和3都是素数，所有只含有质因子2和3的整数（即3-光滑数，如2、3、4、6、8、9……）的倒数在十二进制中都是有限小数。而五个最常用的分数（、、、和）在十二进制中也都有非常简单的表示形式（分别为0.6、0.4、0.8、0.3和0.9）。12是拥有这一性质的最小的底数。在表示分数方面，除了六十进制外，十二进制要比其他常用的进制（诸如十进制、二进制、二十进制、八进制和十六进制）都更为方便。

The duodecimal system (also known as base 12 or dozenal) is a positional notation numeral system using twelve as its base. In this system, the number ten may be written by a rotated "2" (2) and the number eleven by a rotated "3" (3). This notation was introduced by Sir Isaac Pitman. These digit forms are available as Unicode characters on computerized systems since June 2015 as ↊ (Code point 218A) and ↋ (Code point 218B), respectively. Other notations use "A", "T", or "X" for ten and "B" or "E" for eleven. The number twelve (that is, the number written as "12" in the base ten numerical system) is instead written as "10" in duodecimal (meaning "1 dozen and 0 units", instead of "1 ten and 0 units"), whereas the digit string "12" means "1 dozen and 2 units" (i.e. the same number that in decimal is written as "14"). Similarly, in duodecimal "100" means "1 gross", "1000" means "1 great gross", and "0.1" means "1 twelfth" (instead of their decimal meanings "1 hundred", "1 thousand", and "1 tenth").