\substack - Used to create multi-line subscripts or superscripts.
{ \substack}
\substack command draws multi-line subscripts or superscripts.
\sum_{ \substack{ 1\lt i\lt 3 \\ 1\le j\lt 5 }} a_{ij}
^{\substack{\text{a very} \\ \text{contrived} \\ \text{example} }} {\frac ab}_{\substack{ \text{isn't} \\ \text{it?} }}