When connecting links of thread pipe, the thread joint compound should be applied to:

The application of thread joint compound is critical for ensuring a tight seal when connecting threaded pipe links. It is designed primarily to prevent leaks by filling in the small gaps between the threads.

When applying the compound, it should be used on the male threads only. This is because the male threads are the part that will engage with the female threads when the two pieces are screwed together. By applying the compound exclusively to the male threads, you create a seal that helps to prevent any fluid from leaking out along the joint once the connection is made.

This approach also prevents excess compound from entering the pipe, which could cause blockages or contaminations within the system. Properly applying the joint compound to only the threads that are meant to form the connection thus contributes to both the effectiveness of the seal and the overall efficiency of the plumbing system.