Long Left Tack

The Long Left Tack at U+27DE is a mathematical relation symbol, essentially an extended left-facing tack used for specialized turnstile-style notation. It belongs to the Miscellaneous Mathematical Symbols-A block, alongside other uncommon operators and relation marks used in formal writing. Its main real-world use is in mathematical logic, proof theory, and theoretical computer science, where tack and turnstile symbols describe entailment, derivability, satisfaction, or related formal relationships. A left-facing tack variant can be chosen when the direction of the relation matters, such as writing a sequent or judgment in a style where assumptions, conclusions, or semantic conditions are arranged around the symbol. Because the long form is highly specialized, it is much less common than the ordinary turnstile and tack symbols used in textbooks, papers, and proof assistants. It may appear in dense notation designed by a particular author or system, rather than in general arithmetic, programming syntax, or everyday technical writing. Font coverage can be patchier than for more familiar mathematical operators, so it is sometimes substituted with simpler tack notation in plain-text materials.