x is by default interpreted as DoubleBracketingBar[x].
Used in mathematics to indicate taking a norm.
Sometimes used for determinant.
Sometimes used to indicate a matrix.
Not the same as \[DoubleVerticalBar].
Drawn in monospaced fonts with a small left-pointing tee to indicate direction.
See The Mathematica Book: Section 3.10.4.
See also: \[LeftBracketingBar] .