The full point, full stop (Commonwealth English) or period (North American English) is a punctuation mark. It is used for several purposes, the most frequent of which is to mark the end of a declaratory sentence (as opposed to a question or exclamation); this sentence-terminal use is properly, or the precise meaning of, full stop.
The full stop is also often used alone to indicate omitted characters (or in an ellipsis, "..." to indicate omitted words). It may be placed after an initial letter used to stand for a name, or sometimes after each individual letter in an initialism or acronym, for example, "U.S.A."; however, this style is declining, and many initialisms like UK or NATO have individually become accepted norms. A full stop is also frequently used at the end of word abbreviations – in British usage, primarily truncations like Rev., but not after contractions like Revd; however, in American English it is used in both cases.
The full point also has multiple contexts in mathematics and computing, where it may be called a point (short for decimal point) or a dot. The full point glyph is sometimes called a baseline dot because, typographically, it is a dot on the baseline. This term distinguishes it from the interpunct (a raised dot). While full stop technically only applies to the full point when used to terminate a sentence, the distinction – drawn since at least 1897 – is not maintained by all modern style guides and dictionaries.
The full stop symbol derives from the Greek punctuation introduced by Aristophanes of Byzantium in the 3rd century BC, In his system, there were a series of dots whose placement determined their meaning. The full stop at the end of a completed thought or expression was marked by a high dot ⟨˙⟩, called the stigmḕ teleía (στιγμὴ τελεία) or "terminal dot". The "middle dot" ⟨·⟩, the stigmḕ mésē (στιγμὴ μέση), marked a division in a thought occasioning a longer breath (essentially a semicolon) and the low dot ⟨.⟩, called the hypostigmḕ (ὑποστιγμή) or "underdot", marked a division in a thought occasioning a shorter breath (essentially a comma). In practice, scribes mostly employed the terminal dot; the others fell out of use and were later replaced by other symbols. From the 9th century, the full stop began appearing as a low mark instead of a high one; by the advent of printing in Western Europe, the low mark was regular and then universal.
The name period is first attested (as the Latin loanword peridos) in Ælfric of Eynsham's Old English treatment on grammar. There, it is distinguished from the full stop (the distinctio) and continues the Greek underdot's earlier function as a comma between phrases. It shifted its meaning to a dot marking a full stop in the works of the 16th-century grammarians. In 19th-century texts, both British English and American English were consistent in their usage of the terms period and full stop. The word period was used as a name for what printers often called the "full point" or the punctuation mark that was a dot on the baseline and used in several situations. The phrase full stop was only used to refer to the punctuation mark when it was used to terminate a sentence. This distinction seems to be eroding. For example, the 1998 edition of Fowler's Modern English Usage used full point for the character after an abbreviation, but full stop or full point at the end of a sentence;, while the 2015 edition treats them as synonymous (and prefers full stop), and New Hart's Rules does likewise (but prefers full point). The last edition (1989) of the original Hart's Rules exclusively used full point.
Full stops indicate the end of sentences that are not questions or exclamations.
A full stop is used after some abbreviations. If the abbreviation ends a declaratory sentence there is no additional period immediately following the full stop that ends the abbreviation (e.g. "My name is Gabriel Gama, Jr."). Though two full stops (one for the abbreviation, one for the sentence ending) might be expected, conventionally only one is written. This is an intentional omission, and thus not haplography, which is unintentional omission of a duplicate. In the case of an interrogative or exclamatory sentence ending with an abbreviation, a question or exclamation mark can still be added (e.g. "Are you Gabriel Gama, Jr.?").
According to the Oxford A–Z of Grammar and Punctuation, "If the abbreviation includes both the first and last letter of the abbreviated word, as in 'Mister' ['Mr'] and 'Doctor' ['Dr'], a full stop is not used."[better source needed] This does not include, for example, the standard abbreviations for titles such as Professor ("Prof.") or Reverend ("Rev."), because they do not end with the last letter of the word they are abbreviating.
In acronyms and initialisms, the modern style is generally to not use full points after each initial (e.g.: DNA, UK, USSR). The punctuation is somewhat more often used in American English, most commonly with U.S. and U.S.A. in particular. However, this depends much upon the house style of a particular writer or publisher. As some examples from American style guides, The Chicago Manual of Style (primarily for book and academic-journal publishing) deprecates the use of full points in acronyms, including U.S., while The Associated Press Stylebook (primarily for journalism) dispenses with full points in acronyms except for certain two-letter cases, including U.S., U.K., and U.N., but not EU.
The period glyph is used in the presentation of numbers, but in only one of two alternate styles at a time.
In the more prevalent usage in English-speaking countries, the point it represents a decimal separator, visually dividing whole numbers from fractional (decimal) parts. The comma is then used to separate the whole-number parts into groups of three digits each, when numbers are sufficiently large.
The more prevalent usage in much of Europe, southern Africa, and Latin America (with the exception of Mexico due to the influence of the United States), reverses the roles of the comma and point, but sometimes substitutes a space for a point.
India, Bangladesh, Nepal, and Pakistan follow the Indian numbering system, which utilizes commas and decimals much like the aforementioned system popular in most English-speaking countries, but separates values of one hundred thousand and above differently, into divisions of lakh and crore:
In countries that use the comma as a decimal separator, the point is sometimes found as a multiplication sign; for example, 5,2 . 2 = 10,4; this usage is impractical in cases where the point is used as a decimal separator, hence the use of the interpunct: 5.2 · 2 = 10.4. This notation is also seen when multiplying units in science; for example, 50 km/h could be written as 50 km·h−1. However, the point is used in all countries to indicate a dot product, i.e. the scalar product of two vectors.
It is used in many programming languages as an important part of the syntax. C uses it as a means of accessing a member of a struct, and this syntax was inherited by C++ as a means of accessing a member of a class or object. Java and Python also follow this convention. Pascal uses it both as a means of accessing a member of a record set (the equivalent of struct in C), a member of an object, and after the end construct that defines the body of the program. In APL it is also used for generalised inner product and outer product. In Erlang, Prolog, and Smalltalk, it marks the end of a statement ("sentence"). In a regular expression, it represents a match of any character. In Perl and PHP, the dot is the string concatenation operator. In the Haskell standard library, it is the function composition operator. In COBOL a full stop ends a statement.
In file systems, the dot is commonly used to separate the extension of a file name from the name of the file. RISC OS uses dots to separate levels of the hierarchical file system when writing path names—similar to
/ (forward-slash) in Unix-based systems and
\ (back-slash) in MS-DOS-based systems and the Windows NT systems that succeeded them.
Bourne shell-derived command-line interpreters, such as sh, ksh, and bash, use the dot as a command to read a file and execute its content in the running interpreter. (Some of these also offer source as a synonym, based on that usage in the C-shell.)
The term STOP was used in telegrams in place of the full stop. The end of a sentence would be marked by STOP, because "communications was greatly increased during the World War, when the Government employed it widely as a precaution against having messages garbled or misunderstood, as a result of the misplacement or emission of the tiny dot or period."
In British English, the words "full stop" at the end of an utterance strengthen it, it admits of no discussion: "I'm not going with you, full stop." In American English the word "period" serves this function.
The practice in the United States and Canada is to place full stops and commas inside quotation marks in most styles. In the British system, which is also called "logical quotation", full stops and commas are placed according to grammatical sense: This means that when they are part of the quoted material, they should be placed inside, and otherwise should be outside. For example, they are placed outside in the cases of words-as-words, titles of short-form works, and quoted sentence fragments.
There is some national crossover. American style is common in British fiction writing. British style is sometimes used in American English. For example, the Chicago Manual of Style recommends it for fields where comma placement could affect the meaning of the quoted material, such as linguistics and textual criticism.
Use of placement according to logical or grammatical sense, or "logical convention", now the more common practice in regions other than North America, was advocated in the influential book The King's English by Fowler and Fowler, published in 1906. Prior to the influence of this work, the typesetter's or printer's style, or "closed convention", now also called American style, was common throughout the world.
There have been a number of practices relating to the spacing after a full stop. Some examples are listed below:
Although the present Greek full stop (τελεία, teleía) is romanized as a Latin full stop and encoded identically with the full stop in Unicode, the historic full stop in Greek was a high dot and the low dot functioned as a kind of comma, as noted above. The low dot was increasingly but irregularly used to mark full stops after the 9th century and was fully adapted after the advent of print. The teleia should also be distinguished from the ano teleia mark, which is named "high stop" but looks like an interpunct (a middle dot) and principally functions as the Greek semicolon.
In some East Asian languages, notably Chinese and Japanese, a small circle is used instead of a solid dot: "。" (U+3002 "Ideographic Full Stop", simplified Chinese: 句号, traditional Chinese: 句號, Japanese: 句点). Notably, in Taiwan, Hong Kong, and Macao usage, the full stop is written at center height instead of on the line.
In the Devanagari script, used to write Hindi and Sanskrit among other Indian languages, a vertical line ("।") (U+0964 "Devanagari Danda") is used to mark the end of a sentence. It is known as poorna viraam (full stop) in Hindi, and Daa`ri in Bengali. Some Indian languages also use the full stop, such as Marathi. In Tamil, it is known as mutrupulli, which means end dot.
In Sinhala, it is known as kundaliya: "෴" ((U+0DF4) symbol "full stop"). Periods were later introduced into Sinhala script after the introduction of paper due to the influence of Western languages. See also Sinhala numerals.
Urdu uses the "۔" (U+06D4) symbol.
In Thai, no symbol corresponding to the full stop is used as terminal punctuation. A sentence is written without spaces, and a space is typically used to mark the end of a clause or sentence.
In the Ge'ez script used to write Amharic and several other Ethiopian and Eritrean languages, the equivalent of the full stop following a sentence is the ˈarat nettib "።"—which means four dots. The two dots on the right are slightly ascending from the two on the left, with space in between.
The character is encoded at U+002E . FULL STOP (HTML
Researchers from Binghamton University performed a small study, published in 2016, on young adults and found that text messages that included sentences ended with full stops—as opposed to those with no terminal punctuation—were perceived as insincere, though they stipulated that their results apply only to this particular medium of communication: "Our sense was, is that because [text messages] were informal and had a chatty kind of feeling to them, that a period may have seemed stuffy, too formal, in that context," said head researcher Cecelia Klin. The study did not find handwritten notes to be affected.
A 2016 story by Jeff Guo in The Washington Post, stated that the line break had become the default method of punctuation in texting, comparable to the use of line breaks in poetry, and that a period at the end of a sentence causes the tone of the message to be perceived as cold, angry or passive-aggressive.
According to Gretchen McCulloch, an internet linguist, using a full stop to end messages is seen as "rude" by more and more people. She said this can be attributed to the way we text and use instant messaging apps like WhatsApp and Facebook Messenger. She added that the default way to break up one's thoughts is to send each thought as an individual message. 
Punctuation marks are placed inside the quotation marks only if the sense of the punctuation is part of the quotation; this system is referred to as logical quotation.
In the British style (OUP 1983), all signs of punctuation used with words and quotation marks must be placed according to the sense.
The British style is strongly advocated by some American language experts. In defense of nearly a century and a half of the American style, however, it may be said that it seems to have been working fairly well and has not resulted in serious miscommunication. Whereas there clearly is some risk with question marks and exclamation points, there seems little likelihood that readers will be misled concerning the period or comma. There may be some risk in such specialized material as textual criticism, but in that case author and editors may take care to avoid the danger by alternative phrasing or by employing, in this exacting field, the exacting British system. In linguistic and philosophical works, specialized terms are regularly punctuated the British way, along with the use of single quotation marks. [quote attributed to Chicago Manual of style, 14th ed.]
According to what is sometimes called the British style (set forth in The Oxford Guide to Style [the successor to Hart's Rules]; see bibliog. 1.1.]), a style also followed in other English-speaking countries, only those punctuation points that appeared in the original material should be included within the quotation marks; all others follow the closing quotation marks. … In the kind of textual studies where retaining the original placement of a comma in relation to closing quotation marks is essential to the author's argument and scholarly integrity, the alternative system described in 6.10 ['the British style'] could be used, or rephrasing might avoid the problem.