Another recent question was …
Does a negative double of 1♦ after partner opens 1
show both majors?
The standard treatment is that a negative double of 1♦ after partner opens 1 shows both majors, usually 4-4. Longer and unequal lengths such as 5-4 or 4-5 are possible with a weak hand.
Here is why the double needs to be both majors: Due to the way Bridge scoring works, it is extremely important to find your eight-card major suit fits. In a competitive auction you need to do this as quickly as possible. This means that if you have only one major, even if it is only four-cards long, you must bid it right away so partner knows which one you have. When the opponents overcall 1♥, you can use a negative double to show four spades and bid 1 ♠ to show five because, unlike after a 1♦ overcall, there is no ambiguity as to which major your side holds.