Used by opener to show exactly 3 trump support for responder's suit, whereas a direct raise shows 4 trump support.
S: xxxx H: Axxx D: Qxx C: Qx S: x H: KQx D: KJxx C: Axxxx W N E S 1C P 1H 1S X P 2H P/ONotes: South's double is a support (raise) double, showing exactly 3 trumps. If South bid 2H, he would show 4 trumps.