The double bar symbol 
is used to denote certain kinds of norms in mathematics
(e.g., 
or 
). It is also used to denote parallel
lines, as in 
,
and in an older notation for the covariant derivative.
In this work, the notation 
or 
is reserved for matrix and function
norms, respectively.
Double Bar
See also
Bar, Covariant Derivative, Norm, Matrix Norm, Parallel, Single BarExplore with Wolfram|Alpha
References
Bringhurst, R. The Elements of Typographic Style, 2nd ed. Point Roberts, WA: Hartley and Marks, p. 277, 1997.Referenced on Wolfram|Alpha
Double BarCite this as:
Weisstein, Eric W. "Double Bar." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/DoubleBar.html