An AN12 female 37° flare seat on one side and an AN10 male flare on the other make this a neat single-step reducer for larger plumbing. Typical uses are joining an AN12 hose end or tank port to AN10 hardware, or stepping a big cooler or surge tank outlet down to the AN10 line that feeds the rest of the system. Because it drops only one size, the effect on flow is limited, which makes it more tolerable on higher-volume runs than a deeper reduction would be. Ensure both cone faces are clean and free of nicks, as sealing happens entirely on the 37° seat.