In a positional numeral system, the radix (pl.: radices) or base is the number of unique digits, including the digit zero, used to represent numbers. For example, for the decimal system (the most common system in use today) the radix is ten, because it uses the ten digits from 0 through 9.
In any standard positional numeral system, a number is conventionally written as (x)_{y} with x as the string of digits and y as its base, although for base ten the subscript is usually assumed (and omitted, together with the pair of parentheses), as it is the most common way to express value. For example, (100)_{10} is equivalent to 100 (the decimal system is implied in the latter) and represents the number one hundred, while (100)_{2} (in the binary system with base 2) represents the number four.^{[1]}
Radix is a Latin word for "root". Root can be considered a synonym for base, in the arithmetical sense.
In the system with radix 13, for example, a string of digits such as 398 denotes the (decimal) number 3 × 13^{2} + 9 × 13^{1} + 8 × 13^{0} = 632.
More generally, in a system with radix b (b > 1), a string of digits d_{1} … d_{n} denotes the number d_{1}b^{n−1} + d_{2}b^{n−2} + … + d_{n}b^{0}, where 0 ≤ d_{i} < b.^{[1]} In contrast to decimal, or radix 10, which has a ones' place, tens' place, hundreds' place, and so on, radix b would have a ones' place, then a b^{1}s' place, a b^{2}s' place, etc.^{[2]}
Commonly used numeral systems include:
Base/radix | Name | Description |
---|---|---|
2 | Binary numeral system | Used internally by nearly all computers. The two digits are "0" and "1", expressed from switches displaying OFF and ON, respectively. Used in most electric counters. |
8 | Octal system | Used occasionally in computing. The eight digits are "0"–"7" and represent 3 bits (2^{3}). |
10 | Decimal system | Used by humans in the vast majority of cultures. Its ten digits are "0"–"9". Used in most mechanical counters. |
12 | Duodecimal (dozenal) system | Sometimes advocated due to divisibility by 2, 3, 4, and 6. It was traditionally used as part of quantities expressed in dozens and grosses. |
16 | Hexadecimal system | Often used in computing as a more compact representation of binary (1 hex digit per 4 bits). The sixteen digits are "0"–"9" followed by "A"–"F" or "a"–"f". |
20 | Vigesimal system | Traditional numeral system in several cultures, still used by some for counting. Historically also known as the score system in English, now most famous in the phrase "four score and seven years ago" in the Gettysburg Address. |
60 | Sexagesimal system | Originally used in modified form in ancient Sumer and passed to the Babylonians.^{[3]} Used today as the basis of modern circular coordinate system (degrees, minutes, and seconds) and time measuring (minutes, and seconds) by analogy to the rotation of the Earth. |
The octal and hexadecimal systems are often used in computing because of their ease as shorthand for binary. Every hexadecimal digit corresponds to a sequence of four binary digits, since sixteen is the fourth power of two; for example, hexadecimal 78_{16} is binary 1111000_{2}. Similarly, every octal digit corresponds to a unique sequence of three binary digits, since eight is the cube of two.
This representation is unique. Let b be a positive integer greater than 1. Then every positive integer a can be expressed uniquely in the form
where m is a nonnegative integer and the r's are integers such that
Radices are usually natural numbers. However, other positional systems are possible, for example, golden ratio base (whose radix is a non-integer algebraic number),^{[5]} and negative base (whose radix is negative).^{[6]} A negative base allows the representation of negative numbers without the use of a minus sign. For example, let b = −10. Then a string of digits such as 19 denotes the (decimal) number 1 × (−10)^{1} + 9 × (−10)^{0} = −1.