the above statement[s] are true. however, for example, a 1/4-18npt thread can be taped using a 7/16" drill for a through hole and get acceptable results. but you will get superior results using a 27/64" drill and a 1/4npt TAPERED reamer, which as the description implies, DOES make a tapered hole. this method is better because it can be used with or without sealer, and assists the 60* thread angle in doing it's job as designed. the practice of using straight through holes was started as a cost saving manufacturing move when it was discovered acceptable [usable] results could be obtained using a non tapered hole for most applications requiring pipe threads. in the above example, the 7/16" drill size is also the required size for a 1/4-18nps[coupling] thread, allowing use of just a common drill without a reamer operation for both types of pipe threads. i also own several common sizes of taper gages used to ensure the proper hole TAPER is obtained before tapping a tapered pipe thread. these gages have a very pronounced [obvious] taper that can be easily seen by the eye. i used this method daily for 42+ years as a machinist tapping 1/8 through 4"npt threads in natural gas metering devices. upon the advent of cnc machining, the tapered reamer was made obsolete in the smaller [1" or less] sizes as it was very easy to bore a tapered hole of the required dimension needed for the larger sizes, and this hole taper was a mandated check for quality purposes.