I am looking for the name of the next simple fact(theorem):
let $M$ be some matrix and let $k$ be some combinatorial rectangle in $M$. denote the matrix of $k$ by $M_k$. It holds that:
$Rank(M)\ge Rank(M_k)$
(I can easily prove it. But I want my paper to be brief so I just want to quote this theorem in the paper).