National Pipe Thread, a very old standard created in the day of cast iron pipe. 1/4" NPT threads would be found on the outside of a thick walled pipe that had a one quarter inch inside diameter.
With better metalurgy/maunfacturing these days, we can make pipes with thinner walls. Thus a larger inside diameter with the same traditional 1/4" NPT threads.
Doh! beat me to it. Nasty pH Types!
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