What was the difference, during the U.S. Civil War, between an officer’s rank in the volunteer army and the regular army?

by elphin

I’ve been reading about William T. Sherman and came across some details that confuse me. Apparently after Shiloh he was promoted from Brigadier General to Major General in the volunteer army. This was in May, 1862. After the fall of Vicksburg (July 4, 1863*) he was promoted to Brigadier General in the regular army. So, for over a year, he was both a Major General and (I presume he only rose one rank) a Colonel.

I am wondering why there was this disparity in ranks and if it occurred in other wars as well.

* 127 years ago today!

DanKensington

More can be said on the matter - especially if this also happens in other places! No lie, I'm curious myself - but on the specific aspect of American ranks in the Civil War, this previous post with contributions from u/PartyMoses and u/Bacarruda examines the tangle you see.