Abstract: Apparatus for preventing jackknifing between a tractor of a trailer truck and a trailer coupled thereto is disclosed. The apparatus includes a latch block that is movable to a deployed position forward of the front edge of the tractor's fifth wheel and a pair of stop blocks, one disposed on each side of the trailer wear plate so that a laterally extending arm of the latch block will come into contact with one of them upon relative pivotal movement of the fifth wheel and trailer wear plate beyond a predetermined angle. Upon making contact, the blocks prevent further angular movement. Arms of the latch block have projecting end portions on their upper side that slide under end portions of the stop blocks which are spaced apart from the wear plate. This secures the latch block from being sprung downward past the stop block upon occurrence of jackknifing. The latch block is supported by a forward extension of the fifth wheel and by gussets that extend underneath the fifth wheel and are welded thereto.