Conformable matrix
A matrix in mathematics is conformable if its dimensions are suitable for defining some operation (e.g. addition, multiplication, etc.).
In order to be conformable to addition, matrices need to have the same dimension. Thus A, B and C all must have dimension m × n in the equation
for some fixed m and n.
For matrix multiplication, consider the equation
If A has dimension m × n, then B has to have dimension n × p for some p, so that C will have dimension m × p.