Here is a good illustration of what happens when you try to thread a nipple with male NPS threads into a fitting with female NPT threads:
https://www.pbase.com/image/91533235
With tapered threads, as you tighten the joint, ALL the male threads within the fitting compress circumferentially against the female threads they are in contact with. Not only does this result in a minimal leak path between the roots and crests of the threads (which is easily sealed with pipe dope), but it also fixes the male pipe firmly in place within the female fitting.
However, threading male NPS threads into female NPT threads causes three problems:
1. Only a few threads are able to engage.
2. A tight connection only occurs at the first (leading) thread; the remaining threads are a loose fit in the taper. This means the leak path is not just in the tiny gap between thread crest and thread root, but along the entire flanks of the threads.
3. Because all but one of the threads are loose, the nipple/pipe is not firmly located in the fitting. In other words, any pressure exerted perpendicular to the nipple/pipe can cause it to move within the fitting, breaking any dope seal and opening up a new leak path.
https://www.pbase.com/image/91533235
With tapered threads, as you tighten the joint, ALL the male threads within the fitting compress circumferentially against the female threads they are in contact with. Not only does this result in a minimal leak path between the roots and crests of the threads (which is easily sealed with pipe dope), but it also fixes the male pipe firmly in place within the female fitting.
However, threading male NPS threads into female NPT threads causes three problems:
1. Only a few threads are able to engage.
2. A tight connection only occurs at the first (leading) thread; the remaining threads are a loose fit in the taper. This means the leak path is not just in the tiny gap between thread crest and thread root, but along the entire flanks of the threads.
3. Because all but one of the threads are loose, the nipple/pipe is not firmly located in the fitting. In other words, any pressure exerted perpendicular to the nipple/pipe can cause it to move within the fitting, breaking any dope seal and opening up a new leak path.