It's important in the game of doubles to tell your partner what kind of serve you are serving.
The prefered serve is the backhand serve.
For example, one can serve to the 'T' or the front corners.
One can also serve to the back corners called the 'flick serve'. Below is Fu Haifeng doing a flick serve to Tan Boon Heong's backhand corner.
Another variation is the drive serve.
The server should signal to the back player what kind of service he's doing. For example, if the server did a flick serve, he should communicate to his partner via a signal. When doing the flick serve, both the serving side must be prepared to defend a smash and adopt a side by side stance.
For the serve to the front corner, a simple signal will do, so that the back person gets ready to smash. Here Koo Kien Kiat (MAS) signals to his partner Tan Boon Heong (MAS), the direction of his serve.