its mathematical meaning), it is linked to in the entry name. Arithmetic operators Equality, equivalence and similarity Comparison Set theory Basic logic Several logical symbols are widely used in all mathematics, and are listed here. For symbols that are used only in mathematical logic, or are rarely used, see List of logic symbols. Blackboard bold The blackboard bold typeface is widely used for denoting the basic number systems. These systems are often also denoted by the corresponding uppercase bold letter. A clear advantage of blackboard bold is that these symbols cannot be confused with anything else. This allows using them in any area of mathematics, without having to recall their definition. For example, if one encounters in combinatorics, one should immediately know that this denotes the real numbers, although combinatorics does not study the real numbers (but it uses them for many proofs). Calculus (Capital Greek letter delta—not to be confused with , which may denote a geometric triangle or, alternatively, the symmetric difference of two sets.}} (Note: the notation is not recommend for the four-gradient since both and are used to denote the d'Alembertian; see below.) (here an actual box, not a placeholder) Linear and multilinear algebra Advanced group theory Infinite numbers Brackets Many sorts of brackets are used in mathematics. Their meanings depend not only on their shapes, but also on the nature and the arrangement of what is delimited by them, and sometimes what appears between or before them. For this reason, in the entry titles, the symbol is used as a placeholder for schematizing the syntax that underlies the meaning. Parentheses Square brackets Braces Other brackets Symbols that do not belong to formulas In this section, the symbols that are listed are used as some sorts of punctuation marks in mathematical reasoning, or as abbreviations of English phrases. They are generally not used inside a formula. Some were used in classical logic for indicating the logical dependence between sentences written in plain English. Except for the first two, they are normally not used in printed mathematical texts since, for readability, it is generally recommended to have at least one word between two formulas. However, they are still used on a black board for indicating relationships between formulas. Miscellaneous See also Related articles Language of mathematics Mathematical notation Notation in probability and statistics Physical constants Related lists List of mathematical symbols by subject List of logic symbols