When I want to type a vertical bar in LaTeX, I found that I could not find a proper command to display it. Then I realized the problem was not so simple.
What is a vertical bar
The following definition was copied and pasted from Vertical bar - Wikipedia.
The vertical bar(|) is a computer character and glyph with various uses in mathematics, computing, and typography. It has many names, often related to particular meanings: Sheffer stroke(in logic), verti-bar, vbar, stick, vertical line, vertical slash, bar, obelisk, pike, or pipe, and several variants on these names. It is occasionally considered an allograph of broken bar.
How to use it in 
In , the vertical bar can be used as delimiter in mathematical mode. The sequence
\| creates a double vertical line(a | b \| c is set as ).
This has different spacing from
\mid and \parallel, which are relational operators: a \mid b \parallel c is set as . In
text mode, the vertical bar produces an em dash(—). The
\textbar command can be used to produce a vertical bar.
A tip
If you use \textbar text mode, then you may find there is no space between the vertical bar and the following content, like
|following. In this situation, just add a pair of curly brackets to solve this problem: \textbar{} following.
