The Su-27UB is the two seat version of the Su-27, officially designated T-10U. The Flanker-C has a tandem cockpit, in which the back seater sits higher than the front seater, and an enlarged spine to accommodate the larger cockpit. The vertical stabilisers are higher due to an extension at the bottom, and this is why the rudder do not extend all the way to the bottom of them. Su-27UBK is the designation of the export version of the Su-27UB. It is only equipment-wise different from the Russian version.