Typographical conventions in mathematical formulae

International recommendations

Various international authorities, including IUPAC, NIST and ISO have produced similar recommendations with regard to typesetting variables and other mathematical symbols (whether in equations or otherwise).

In general, anything that represents a variable (for example, h for a patient's height) should be set in italic, and everything else should be set in roman type. This applies equally to characters from the Latin/English alphabet (a, b, c, ...; A, B, C, ...) as to letters from any other alphabet, most notably Greek (α, β, γ, ...; Α, Β, Γ, ...). Any operator, such as cos (representing cosine) or ∑ (representing summation), should therefore be set roman. Note that each element must be set depending upon its own merits, including subscripts and superscripts. Thus, h_i would be suitable for the initial height, while h_i would represent one instance from a set of heights (h₁, h₂, h₃, ...). Notice that numbers (1, 2, 3, etc.) are not variables, and so are always set roman. Likewise, in some special cases symbols are used to represent general constants, such as π used to represent the ratio of a circle's circumference to its diameter, and such general constants can be set in roman. (This does not apply to parameters which are merely chosen to not vary.)

For vectors, matrices and tensors, it is recommended to set the variable itself in boldface (excluding any associated subscripts or superscripts). Hence, u_i would be suitable for the initial velocity, while u_i would represent one instance from a set of velocities (u₁, u₂, u₃, ...). Italic is still used for variables, both for lowercase and for uppercase symbols (Latin, Greek, or otherwise). The only general situation where italic is not used for bolded symbols is for vector operators, such as ∇ (nabla), set bold and roman.

General rules in American mathematical typography

The rules of mathematical typography differ from country to country; thus, American mathematical journals and books will tend to use slightly different conventions from those of European journals.

One advantage of mathematical notation is its modularity—it is possible to write extremely complicated formulae involving multiple levels of super- or subscripting, and multiple levels of fraction bars. However, it is considered poor style to set up a formula in such a way as to leave more than a certain number of levels; for example, in non-math publications

might be rewritten as

(Even in mathematical publications, where 3 or 4 levels of indices are frequent, avoiding multilevel fractions is productive.)

Incidentally, the above formula demonstrates the American rule that italic type is used for all letters representing variables and parameters except uppercase Greek letters, which are in upright type. Upright type is also standard for digits and punctuation; currently, the ISO-mandated style of using upright for constants (such as e, i) is not widespread. Bold Latin capital letters usually represent matrices, and bold lowercase letters are often used for vectors. The names of well-known functions, such as sin x (the trigonometric function sine) and exp x (the constant e raised to the power of x) are written in lowercase upright letters (and often, as shown here, without parentheses around the argument).

Certain important constructs are sometimes referred to by blackboard-bold letters. For example, some authors denote the set of natural numbers by N . Similarly, the symbols Z , Q , and R , may be used to denote the integers, rationals, and reals, respectively. But, as its name suggests, blackboard bold simulates the practice used on chalkboards to indicate boldface. So many non-mathematical publications, having boldface available, use it. Thus, for instance, the integers would be denoted by Z . (In context of math, font variations such as bold/non-bold may encode an arbitrary relation between symbols; using specialized symbols for N etc. allows the author more freedom of expressing such relations.)

Donald Knuth's TeX typesetting engine incorporates a large amount of additional knowledge about American-style mathematical typography.