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Doubled vertical bars measuring the size of a vector.
Also known as: Double bars, Vector norm, Length bars, Magnitude bars
Two vertical bars on each side denote a norm, the length of a vector or the size of a function. Single bars mean absolute value, and the doubling is the only thing distinguishing a vector's length from a number's magnitude on the page.
Denotes a norm: the length of a vector, or a corresponding measure of size for a matrix or a function.
Contexts: Mathematics
Is distinguished from absolute value only by the doubling of the bars, which is why the notation is easily misread at small sizes.
Contexts: Mathematics
Carries a subscript naming which norm is meant when more than one is in play, since a vector has many.
Contexts: Mathematics
Two pairs of tall parallel upright strokes, one pair on each side of an expression.
Single bars, doubled bars and determinant bars all flank an expression.