Important related terms to know:

Kingpin (axle) Pin around which a steer axle's wheels pivot.  

Kingpin (trailer) Anchor pin at the center of a semi trailer's upper coupler which is captured by the locking jaws of a tractor's fifth wheel to attach the tractor to the semi trailer. Use the double handled fifth wheel puller to pull the pin.

Setback Axle Front steering axle moved rearward from the generally accepted standard position. Advantages: Shorter turning radius and more of a vehicle's weight shifted to front axle. 

Sliding Fifth Wheel Fifth wheel mounted to a mechanism that allows it to be moved back and forth for the purpose of adjusting the distribution of weight on the tractor's axles. Also provides the capability to vary vehicle combination lengths.

Sliding Tandem (Slider) Mechanism that allows a tandem axle suspension to be moved back and forth at the rear of a semi trailer, for the purpose of adjusting the distribution of weight between the axles and fifth wheel.

Spread Axle (Spread Tandem) Tandem axle assembly spaced further apart than the standard spacing of 54 inches. The U.S. federal bridge formula favors trailer axles with an eight or nine foot spread by allowing higher weight than on tandems with standard spacing. 

Cargo Bar - Keeps loads from shifting with ratcheting trigger, tightens load with very little effort and is quick to adjust. Use with shackle and lock for security.