# Turnstile (symbol)

In mathematical logic and computer science the symbol ${\displaystyle \vdash }$ has taken the name turnstile because of its resemblance to a typical turnstile if viewed from above. It is also referred to as tee and is often read as "yields", "proves", "satisfies" or "entails". The symbol was first used by Gottlob Frege in his 1879 book on logic, Begriffsschrift.[1]

In TeX, the turnstile symbol ${\displaystyle \vdash }$ is obtained from the command \vdash. In Unicode, the turnstile symbol () is called right tack and is at code point U+22A2.[2] (Code point U+22A6 is named assertion sign ().) On a typewriter, a turnstile can be composed from a vertical bar (|) and a dash (–). In LaTeX there is a turnstile package which issues this sign in many ways, and is capable of putting labels below or above it, in the correct places.[3]

## Interpretations

The turnstile represents a binary relation. It has several different interpretations in different contexts:

• In epistemology, Per Martin-Löf (1996) analyzes the ${\displaystyle \vdash }$ symbol thus: "...[T]he combination of Frege's Urteilsstrich, judgement stroke [ | ], and Inhaltsstrich, content stroke [—], came to be called the assertion sign."[4] Frege's notation for a judgement of some content A
${\displaystyle \vdash A}$
I know A is true.[5]
In the same vein, a conditional assertion
${\displaystyle P\vdash Q}$
From P, I know that Q
${\displaystyle P\vdash Q}$
means that Q is derivable from P in the system.
Consistent with its use for derivability, a "⊢" followed by an expression without anything preceding it denotes a theorem, which is to say that the expression can be derived from the rules using an empty set of axioms. As such, the expression
${\displaystyle \vdash Q}$
means that Q is a theorem in the system.
• In proof theory, the turnstile is used to denote "provability" or "derivability". For example, if T is a formal theory and S is a particular sentence in the language of the theory then
${\displaystyle T\vdash S}$
means that S is provable from T.[7] This usage is demonstrated in the article on propositional calculus. The syntactic consequence of provability should be contrasted to semantic consequence, denoted by the double turnstile symbol ${\displaystyle \models }$. One says that ${\displaystyle S}$ is a semantic consequence of ${\displaystyle T}$, or ${\displaystyle T\models S}$, when all possible valuations in which ${\displaystyle T}$ is true, ${\displaystyle S}$ is also true. For propositional logic, it maybe be shown that semantic consequence ${\displaystyle \models }$ and derivability ${\displaystyle \vdash }$ are equivalent to one-another. That is, propositional logic is sound (${\displaystyle \vdash }$ implies ${\displaystyle \models }$) and complete (${\displaystyle \models }$ implies ${\displaystyle \vdash }$)[8]
• In the typed lambda calculus, the turnstile is used to separate typing assumptions from the typing judgment.[9][10]
• In category theory, a reversed turnstile (${\displaystyle \dashv }$), as in ${\displaystyle F\dashv G}$, is used to indicate that the functor F is left adjoint to the functor G.[11] More rarely, a turnstile (${\displaystyle \vdash }$). as in ${\displaystyle F\vdash G}$, is used to indicate that the functor F is right adjoint to the functor G.[12]
• In APL the symbol is called "right tack" and represents the ambivalent right identity function where both XY and ⊢Y are Y. The reversed symbol "⊣" is called "left tack" and represents the analogous left identity where XY is X and ⊣Y is Y.[13][14]
• In combinatorics, ${\displaystyle \lambda \vdash n}$ means that λ is a partition of the integer n.[15]
• In Hewlett-Packard's HP-41C/CV/CX and HP-42S series of calculators, the symbol (at code point 127 in the FOCAL character set) is called "Append character" and is used to indicate that the following characters will be appended to the alpha register rather than replacing the existing contents of the register. The symbol is also supported (at code point 148) in a modified variant of the HP Roman-8 character set used by other HP calculators.

## Similar graphemes

• (U+A714) Modifier Letter Mid Left-Stem Tone Bar
• (U+251C) Box Drawings Light Vertical And Right
• (U+314F) Korean Ah
• Ͱ (U+0370) Greek Capital Letter Heta
• ͱ (U+0371) Greek Small Letter Heta
• (U+2C75) Latin Capital Letter Half H
• (U+2C76) Latin Small Letter Half H
• (U+23AB) Right curly bracket

## Notes

1. ^ Frege 1879
2. ^ Unicode standard
3. ^ http://www.ctan.org/tex-archive/macros/latex/contrib/turnstile
4. ^ Martin-Löf 1996, pp. 6, 15
5. ^ Martin-Löf 1996, p. 15
6. ^ Chapter 6, Formal Language Theory
7. ^ Troelstra & Schwichtenberg 2000
8. ^ Dirk van Dalen, Logic and Structure (1980), Springer, ISBN 3-540-20879-8. See Chapter 1, section 1.5.
9. ^ Peter Selinger, Lecture Notes on the Lambda Calculus
10. ^ Schmidt 1994
11. ^ adjoint functor in nLab
12. ^ @FunctorFact (5 July 2016). "Functor Fact on Twitter" (Tweet) – via Twitter.
13. ^ Iverson, APL dictionary
14. ^ Iverson 1987
15. ^ Stanley, Richard P. (1999). Enumerative Combinatorics. Vol. 2 (1st ed.). Cambridge: Cambridge University Press. p. 287.