Both pipe dope and tape attempt to achieve the same purpose. They are both used to LUBRICATE the threads so that they can be inserted farther and tighter into the fitting, thus melding the male and female threads together more.
If you use no tape or dope, you can still crew the two fittings together, but they do won't meld as much to each other due to friction causing too much heat. Tape and dope help reduce the friction, allowing the THREADS to seal better.
Both dope and tape are fine, and they also form a secondary seal at the joint where the threads end at the pipe (see below).
Now, on old metallic pipe that is eroding/corroding, pits will develop in the piping and if you are screwing a male thread into an old female fitting, it may be better to use dope because it can get in the pits better than tape. I have also seen people use fibrous twine in connection with dope to help seal really old metallic piping.
There is really no reason to use BOTH dope and tape unless it makes you feel more comfortable with your work.
Pipe with dope/tape before entering a fitting is on the left. On the right is what happens when the male thread is screwed into a female fitting. The tape/dope does NOT stay on the male threads. It squeezes outside of the joint. If one were to put dope on the female threads, it could be pushed into the fitting causing debris to enter the pipe.
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