Decimal mark

History

In the Middle Ages, before printing, a bar ( ¯ ) over the units digit was used to separate the integral part of a number from its fractional part, e.g. 9995 (meaning, 99.95 in decimal point format). This practice derived from the decimal system used in Indian mathematics and was popularized by the Persian mathematician Al-Khwarizmi, when Latin translation of his work on the Indian numerals introduced the decimal positional number system to the Western world. His Compendious Book on Calculation by Completion and Balancing presented the first systematic solution of linear and quadratic equations in Arabic. A similar notation remains in common use as an underbar to superscript digits, especially for monetary values without a decimal mark, e.g. 99⁹⁵. Later, a "separatrix" (a short, roughly vertical ink stroke) between the units and tenths position became the norm among Arab mathematicians, e.g. 99ˌ95. When this character was typeset, it was convenient to use the existing comma (99,95) or full stop (99.95) instead. The separatrix was also used in England as an L-shaped or vertical bar (|) before the popularization of the period and mid dot decimal points.

Gerbert of Aurillac marked triples of columns with an arc (called a "Pythagorean arc") when using his Hindu–Arabic numeral-based abacus in the 10th century. Fibonacci followed this convention when writing numbers such as in his influential work Liber Abaci in the 13th century.

In France, the full stop was already in use in printing to make Roman numerals more readable, so the comma was chosen. Many other countries, such as Italy, also chose to use the comma to mark the decimal units position. It has been made standard by the ISO for international blueprints. However, English-speaking countries took the comma to separate sequences of three digits. In some countries, a raised dot or dash (upper comma) may be used for grouping or decimal mark; this is particularly common in handwriting.

In the United States, the full stop or period (.) was used as the standard decimal mark.

In the nations of the British Empire, although the full stop could be used in typewritten material and its use was not banned, the interpunct (also known as the decimal point, point, or mid dot) was preferred for the decimal mark in printing technologies that could accommodate it, e.g. 99·95. However, as the mid dot was already in common use in the mathematics world to indicate multiplication, the SI rejected its use as the decimal mark. During the beginning of British metrication in the late 1960s and with impending currency decimalisation, there was widespread debate in the United Kingdom as to whether the decimal comma or decimal point should be preferred: the British Standards Institution and some sectors of industry advocated the comma and the Decimal Currency Board advocated for the point. In the event, the point was decided on by the Ministry of Technology in 1968.

When South Africa adopted the metric system, it adopted the comma as its decimal mark, although a number of house styles, including some English-language newspapers such as The Sunday Times, continue to use the full stop.

The three most spoken international auxiliary languages, Ido, Esperanto, and Interlingua, all use the comma as the decimal mark. Interlingua has used the comma as its decimal mark since the publication of the Interlingua Grammar in 1951. Esperanto also uses the comma as its official decimal mark, while thousands are separated by non-breaking spaces: 12 345 678,9. Ido's Kompleta Gramatiko Detaloza di la Linguo Internaciona Ido (Complete Detailed Grammar of the International Language Ido) officially states that commas are used for the decimal mark while full stops are used to separate thousands, millions, etc. So the number 12,345,678.90123 (in American notation) for instance, would be written 12.345.678,90123 in Ido. The 1931 grammar of Volapük by Arie de Jong uses the comma as its decimal mark, and (somewhat unusually) uses the middle dot as the thousands separator (12·345·678,90123).

In 1958, disputes between European and American delegates over the correct representation of the decimal mark nearly stalled the development of the ALGOL computer programming language. ALGOL ended up allowing different decimal marks, but most computer languages and standard data formats (e.g. C, Java, Fortran, Cascading Style Sheets (CSS)) specify a dot.

The 22nd General Conference on Weights and Measures declared in 2003 that "the symbol for the decimal marker shall be either the point on the line or the comma on the line". It further reaffirmed that "numbers may be divided in groups of three in order to facilitate reading; neither dots nor commas are ever inserted in the spaces between groups" (e.g. 7009100000000000000♠1000000000). This usage has therefore been recommended by technical organizations, such as the United States' National Institute of Standards and Technology.

Digit grouping

For ease of reading, numbers with many digits may be divided into groups using a delimiter. In some countries, these "digit group separators" are only employed to the left of the decimal mark; in others, they are also used to separate numbers with a long fractional part. An important reason for grouping is that it allows rapid judgement of the number of digits, via subitizing (telling at a glance) rather than counting – contrast 7009100000000000000♠1000000000 with 1000000000 for a short billion.

Since 2003, the use of spaces as separators (for example: 7004200000000000000♠20000 and 7006100000000000000♠1000000 for "twenty thousand" and "one million") has been officially endorsed by SI/ISO 31-0 standard, as well as by the International Bureau of Weights and Measures and the International Union of Pure and Applied Chemistry, the American Medical Association's widely followed AMA Manual of Style, and the Metrication Board, among others.

The groups created by the delimiters tend to follow the use of the local language, which varies. In European languages, large numbers are read in groups of thousands and the delimiter (which occurs every three digits when it is used) may be called a "thousands separator". In East Asian cultures, particularly China, Japan, and Korea, large numbers are read in groups of myriads (10,000s) but the delimiter commonly separates every three digits. The Indian numbering system is somewhat more complex: it groups the rightmost three digits in a similar manner to European languages but then groups every two digits thereafter: 1.5 million would accordingly be written 15,00,000 and read as "15 lakh".

The convention for digit group separators historically varied among countries, but usually seeking to distinguish the delimiter from the decimal mark. Traditionally, English-speaking countries employed commas as the delimiter – 10,000 – and other European countries employed periods or spaces: 10.000 or 7004100000000000000♠10000. Because of the confusion that could result in international documents, in recent years the use of spaces as separators has been advocated by the superseded SI/ISO 31-0 standard, as well as by the International Bureau of Weights and Measures and the International Union of Pure and Applied Chemistry, which have also begun advocating the use of a "thin space" in "groups of three". Within the United States, the American Medical Association's widely followed AMA Manual of Style also calls for a thin space. In some online encoding environments (for example, ASCII-only) a thin space is not practical or available, in which case a regular word space or no delimiter are the alternatives. In 2014, the Metrication Board also proposed the system of using a space for separation in the United Kingdom and, while it is not universally adopted, it is standard within the country's construction industry.

Data versus mask

Digit group separators can occur either as part of the data or as a mask through which the data is displayed. This is an example of the separation of presentation and content. In many computing contexts, it is preferred to omit digit group separators from the data and instead overlay them as a mask (an input mask or an output mask). Common examples include spreadsheets and databases in which currency values are entered without such marks but are displayed with them inserted. Similarly, phone numbers can have hyphens or spaces as a mask rather than as data.

In some programming languages, it is possible to group the digits in the program's source code to make it easier to read; see Integer literal: Digit separators. Ada, D, Swift, Java, Perl and Ruby use the underscore (_) character for this purpose. All these languages allow seven hundred million to be entered as 700_000_000. Fixed-form Fortran ignores whitespace (in all contexts), so 700 000 000 is permissible. C++14 allows the use of an apostrophe for digit grouping, so 700'000'000 is permissible.

Exceptions to digit grouping

The International Bureau of Weights and Measures states that "when there are only four digits before or after the decimal marker, it is customary not to use a space to isolate a single digit". Likewise, some manuals of style state that thousands separators should not be used in normal text for numbers from 1000 to 9999 inclusive where no decimal fractional part is shown (in other words, for four-digit whole numbers), whereas others use thousands separators, and others use both. For example, APA style stipulates a thousands separator for "most figures of 1,000 or more" except for page numbers, binary digits, temperatures, etc.

There are always common-sense exceptions to digit grouping, such as years, postal codes and ID numbers of predefined nongrouped format, which style guides usually point out.

In non-base-10 numbering systems

In binary (base-2), a full space can be used between groups of four digits, corresponding to a nibble, or equivalently to a hexadecimal digit. For integer numbers, dots are used as well to separate groups of four bits. Alternatively, binary digits may be grouped by threes, corresponding to an octal digit. Similarly, in hexadecimal (base-16), full spaces are usually used to group digits into twos, making each group correspond to a byte. Additionally, groups of eight bytes are often separated by a hyphen.

Influence of calculators and computers

In countries with a decimal comma, the decimal point is also common as the "international" notation because of the influence of devices, such as electronic calculators, which use the decimal point. Most computer operating systems allow selection of the decimal mark and programs that have been carefully internationalized will follow this, but some programs ignore it and a few may even fail to operate if the setting has been changed.

Countries using Arabic numerals with decimal point

Countries where a dot "." is used as decimal mark include

Countries using Arabic numerals with decimal comma

Countries where a comma "," is used as decimal mark include

Other numeral systems

Unicode defines a decimal separator key symbol (⎖ in hex U+2396, decimal 9110) which looks similar to the apostrophe. This symbol is from ISO/IEC 9995 and is intended for use on a keyboard to indicate a key that performs decimal separation.

In the Arab world, where Eastern Arabic numerals are used for writing numbers, a different character is used to separate the integer and fractional parts of numbers. It is referred to as an Arabic decimal separator (٫) (in hex U+066B) in Unicode. An Arabic thousands separator (٬) also exists.

In Persian, the decimal mark is called momayyez, which is written like a forward slash—there is a small difference between the upside-down (Persian) comma character used in sentences and the Latin comma (٫) (in hex U+066B) used to separate sequences of three digits. To separate sequences of three digits, a Latin comma or blank space may be used; however this is not a standard.

In English Braille, the decimal point, ⠨, is distinct from both the comma, ⠂, and the full stop, ⠲.

Examples of use

The following examples show the decimal mark and the thousands separator in various countries that use the Arabic numeral system.

Unicode characters

Used with Western Arabic numerals (0123456789):

Used with Eastern Arabic numerals (٠١٢٣٤٥٦٧٨٩):

Used with keyboards: