If  $A\text{\hspace{0.17em}\hspace{0.17em}and\hspace{0.17em}\hspace{0.17em}}B$ are two matrices such that  $A+B$ and  $AB$ are both defined, then 

$A,B$  are square matrices not necessarily of same order

$A,B$ are square matrices of same order.

$A\text{\hspace{0.17em}\hspace{0.17em}and\hspace{0.17em}\hspace{0.17em}}B$ can be any matrices

Number of columns of  $A=$number of columns of  $B$