\arrowvert as a middle delimiter. For example in the definition of sets, i.e.
\lbrace x \arrowvert x < a \rbrace. Moreover, I try to avoid
\middle but use the AMSmath macros
\Bigl, ... to set the size explicitly.
If I need the delimiters in "normal" size, I do not use any macro, but then I notice a different horizontal (!) spacing. Consider the case in that
\middle can be used without effect, because all enclosed symbols have at most normal size. Then
\lbrace x \arrowvert x < a \rbrace \left\lbrace x \middle\arrowvert x < a \right\rbrace
look the same vertically, but there is a little bit more horizontal space of both sides of the middle delimiter. Actually, the visual appearance of the latter looks more correct.
The same amount of additional space around the middle delimiter is inserted by the AMSmath keywords
But what shall I do, if I need the normally sized delimiters but cannot use the triplet
\right because there is some over-sized content?
Using no macro as suggested by the AMSmath manual is no option, because then the additional space around the middle delimter is missing. Unfortunately, there does not seem to be something like a
\normalr triplet that does no scaling but fits the horizontal spacing of the other macros.