The Double Matrix Method is used for sets in which there are two variables, each of which has two or more categories. For example, suppose in some group (a company, a school, etc.), one variable is general (male & female), and the other variable can be anything (A & B & C). Then we set up a "matrix:"

The numbers about individual entries (e.g. men in B) can go into the table. Totals for the columns are written at the bottom, and totals for the rows are written at the right. The grand total, in the lower right corner, is everyone in the group, and it must be the sum of the bottom row as well as the sum of the right column.