A seat structure with two, lateral-side latching-unlatching mechanisms for enabling smoothly operating releasable locking and unlocking actions between a seat-base and an inclinable seat-back hinged to the seat-base in the seat structure. Two interengageable arcs of gear teeth forming part of each mechanism engage and release selectively under rocker-lever seat-occupant control to lock and unlock the seat-back for inclinational fixation and adjustment, and, as determined by tooth-pitch in the arcs, enable positive locking to take place in a large number of specific, small-angular-difference, seat-back inclinations. A floating cross-shaft, and two rocker cams joined thereto, interconnect the two lateral side mechanisms. The seat-base is fully twist-deformable in a catastrophic event replicating a conventional rack-and-roll test without the seat-back becoming unlocked relative to the seat-base.