This paper treats linear algebra over a semiring pair, in a wide range of applications to tropical algebra and related areas such as hyperrings and fuzzy rings. First we present a more general category of pairs with their morphisms, called ``weak morphisms,'' paying special attention to supertropical pairs, hyperpairs, and the doubling functor. Then we turn to matrices and the question of whether the row rank, column rank, and submatrix rank of a matrix are equal. Often the submatrix rank is less than or equal to the row rank and the column rank, but there is a counterexample to equality, discovered some time ago by the second author, which we provide in a more general setting (``pairs of the second kind'') that includes the hyperfield of signs. Additional positive results include a version of Cramer's rule, and we find situations when equality holds, encompassing results by Akian, Gaubert, Guterman, Izhakian, Knebusch, and Rowen. We pay special attention to the question of whether $n+1$ vectors of length $n$ need be dependent. At the end, we introduce a category with stronger morphisms, that preserve a surpassing relation.