# what is the use of question mark postindices, how it changes contractions and symmetry properties of indices?

+1 vote

A single question mark indicates a pattern (or wildcard), so that should only be used in e.g. substitute. Under most circumstances you don't need them for normal indices. Two question marks indicate an object wildcard, that is, a wildcard which matches an entire tensor. Have a look at

- What is the meaning and use of Q::.?
- Why the first terms of the expression following the Q::. assignation are not shown?
- You used a \Q and a Q in the mentioned expression. Where are defined and what's their difference?