When coupling a tractor to a semitrailer, how should the tractor be positioned before backing up to the trailer?

AAMVA CDL Manual, Section 6.4.1, Step 3, p. 6-8

Backing in at an angle is one of the most common coupling mistakes new drivers make, and it can snap or bend the landing gear, putting the trailer out of service and creating a safety hazard. Lining up straight in front of the trailer before backing keeps the kingpin and fifth wheel aligned and prevents sideways force on the trailer's support legs.