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. Extensible character. See The Mathematica Book: Section 3.10.4. See also: \[LeftBracketingBar].