Most popular variation:

After partner opens, and RHO overcalls, a negative double shows the other two suits. However, over 1(minor)-1(major) a double guarantees the other major and less so on the other minor.

"Expert" variation:

After 1(minor)-1(major), a double shows 4+ of the other major (nothing about the other minor). After 1C-1D a double shows at least 4-3 in the majors. And after 1H-1S a double shows 4-4 in the minors.

A very different variation:

After 1(minor)-1(major), a double DENIES the other 4 card major. Also after 1C-1D a double DENIES a 4 card major.