Backtick

The backtick is a typographical mark used mainly in computing. It is also known as backquote, grave, or grave accent. The character was designed for typewriters to add a grave accent to a (lower-case ) base letter, by overtyping it atop that letter. On early computer systems, however, this physical dead key+overtype function was rarely supported, being functionally replaced by precomposed characters. Consequently, this ASCII symbol was rarely (if ever) used in computer systems for its original aim and became repurposed in computer programming for many unrelated uses. The sign is located on the left-top of a US or UK layout keyboard, next to the key. Provision (if any) of the backtick on other keyboards varies by national keyboard layout and keyboard mapping. In layouts that use the dead key concept, the backtick key is most frequently chosen for that purpose.

The backtick is a typographical mark used mainly in computing. It is also known as backquote, grave, or grave accent. The character was designed for typewriters to add a grave accent to a (lower-case ) base letter, by overtyping it atop that letter. On early computer systems, however, this physical dead key+overtype function was rarely supported, being functionally replaced by precomposed characters. Consequently, this ASCII symbol was rarely (if ever) used in computer systems for its original aim and became repurposed in computer programming for many unrelated uses. The sign is located on the left-top of a US or UK layout keyboard, next to the key. Provision (if any) of the backtick on other keyboards varies by national keyboard layout and keyboard mapping. In layouts that use the dead key concept, the backtick key is most frequently chosen for that purpose. ==History== ===Typewriters=== On typewriters designed for languages that routinely use diacritics (accent marks), there are two possible solutions. Keys can be dedicated to pre-composed characters or alternatively a dead key mechanism can be provided. With the latter, a mark is made when a dead key is typed but, unlike normal keys, the paper carriage does not move on and thus, the next letter to be typed is printed under the accent. ===Incorporation into ISO 646 and ASCII=== The incorporation of the grave symbol into ASCII is a consequence of this prior existence on typewriters. This symbol did not exist independently as a type or hot-lead printing character. Thus, ISO 646 was born and the ASCII standard updated to include the backtick and other symbols. ===As surrogate of apostrophe or (opening) single quote=== Some early ASCII peripherals designed the backtick and apostrophe to be mirror images of each other: and . This allowed them to be used as matching pairs of open and close quotes while still being somewhat usable as grave and acute accents, made apostrophes typographically correct, and allowed the apostrophe to be used as a prime. This had a number of problems that led most modern systems and Unicode to render the apostrophe as a "straight" one: Open quote is not typographically correct: the correct form ( ) is flipped vertically from what is shown here. No matching double quotes, although two single quotes looked acceptable on proportionally-spaced devices. Wrong appearance if used as overprinted diacritics. Lots of software and documents used the apostrophe for opening as well as for closing quotes. This can still be seen in documents and email from that time (before 1990), and in output generated by some UNIX console programs such as man pages. Institutions that traditionally had used it have abandoned or deprecated it. ==='Dead key' operation=== On computer keyboards with a suitable keyboard layout ('keyboard mapping'), the acts as a dead key. When pressed, it has no immediate effect its effect is to apply a diacritic to the next key pressed. In the UK extended and the US-International layouts, the sequence adds a grave accent (e.g., produces ). In the New Zealand "NZ Aotearoa" layout, the sequence adds a macron (e.g., produces ). const name = "Mary", pet = "lamb"; // Set variables let temp = `${name} has a little ${pet}!`; console.log(temp); // => "Mary has a little lamb!"; Lisp macro systems: The backtick character (called in Scheme and Racket) introduces a quoted expression in which comma-substitution may occur. It is identical to the plain quote, except that a nested expression prefixed with a comma is replaced with the value of that nested expression. If the nested expression happens to be a symbol (that is, a variable name in Lisp), the symbols' value is used. If the expression happens to be program code, the first value returned by that code is inserted at the respective location instead of the comma-prefixed code. This is roughly analogous to the Bourne shell's variable interpolation with $ inside double quotes. Julia: Backticks make a command object, Cmd , that can be run, with run function, like run(`echo Hello world!`) . You can interpolate Julia variables, but only indirectly shell environment variables. m4: A backtick together with an apostrophe quotes strings (to suppress or defer macro expansion). MySQL/MariaDB: A backtick in queries is a delimiter for column, table, and database identifiers. OCaml: The backtick indicates polymorphic variants. Pico: The backtick indicates comments in the programming language. PowerShell: The backtick is used as the escape character. For example, a newline character is denoted `n . Most common programming languages use a backslash as the escape character (e.g., \n ), but because Windows allows the backslash as a path separator, it is impractical for PowerShell to use backslash for a different purpose. Two backticks produce the ` character itself. For example, the nullable boolean of .NET is specified in PowerShell as [Nullable``1[System.Boolean . Python: Prior to version 3.0, backticks were a synonym for the repr() function, which converts its argument to a string suitable for a programmer to view. However, this feature was removed in Python 3.0. Backticks also appear extensively in the reStructuredText plain text markup language (implemented in the Python docutils package). R: The backtick is used to surround non-syntactic variable names. This includes variable names containing special characters or reserved words, among others. Scala: An identifier may also be formed by an arbitrary string between backticks. The identifier then is composed of all characters excluding the backticks themselves. Tom: The backtick creates a new term or to calls an existing term. Unlambda: The backtick character denotes function application. Verilog HDL: The backtick is used at the beginning of compiler's directives. ==Notes== ==References==

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