A,B, P etc

[BigBill]

Does anyone know or have a link that explains what exactly the values for A,B, F, P etc represent in a sight reduction?

 I was asked recently and I know anything with H has to do with altitudes and the Zs are azimuths but the rest I don't know with enough certainty to pass on.

Thanks

"BigBill, .... Unless those letters represent angles and sides of the spherical triangles ( which I think they are), I don't know any other instance where I saw those letters regrading sight reduction."

Thanks. That's what I was thinking but I didn't want to pass on bad information. These are the values being looked up in the Nautical almanac.