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.
Ask your partner how they would feel looking at a 2-4-3-4 pattern after these auctions if the negative double promised only one (unknown) major :
1 1♦ Double 2♦ 11♦ Double 3♦
You can make a negative double with a 5-4 or 4-5 in the majors on two types of hands: First, a hand which is so weak that it can only take one bid (usually less than eight points). Second, a hand that would be stuck for a rebid after biddng the five-card major if the auction were to continue with a diamond raise:
1 1♦ 1 Major 2♦
Pass Pass ??
In other words, a hand that is not strong enough to bid the five-card major and then reopen with double (showing a good 10 points or more) might prefer double followed by bidding the five-card major, which shows a maximum of a poor 10 points. Most often the hand that makes a negative double with nine cards in the majors will have four spades and five hearts, because bidding 1♥ and following with 2♠ is a game force. On the other hand, bidding 1♠ and then reopening with 2♥ can be done on as little as an attractive eight points.