The emphasis is on finding a major suit fit.

When there is one unbid major, doubling shows 4-cards in that suit.

When there are two unbid majors, doubling shows 4 cards in both.

When both majors have been bid, doubling shows both minors.

When the overcall does not take away major suit bidding space, we can leverage the option of doubling.

Also, remember that if partner bids a suit that your double has indicated, then that is a limit bid, not a new suit.