[ethbr0]

It's probably a commentary on the idea that the tapered male diameter is its max diameter(?), versus straight is throughout.

Consequently, a tapered male will fully screw into a straight female, albeit with gaps between the threads at the deep end of the socket.

Whereas a straight male will at most only screw the first few threads into a tapered female, leaving most of the male hanging out.

[johnwalkr]

Tapered pipe threads are actually much larger than you’d expect given the nominal diameter. 1/4 inch tapered pipe threads have a maximum outer diameter of more than 1/2 inch and 18 threads per inch. They will sort of work with a 1/4 inch straight thread but not by fully screwing in.

[lazide]

It’s asking to rip the threads out of the fitting (and have done so when trying). One way you’ll get some seal first, the other you won’t.