What is a 'converter dolly' in a doubles combination?

A converter dolly (sometimes called a 'dolly' or 'A-dolly') is a small wheeled platform with one or two axles, a tongue (draw bar) that connects to the rear of the lead trailer with a pintle hook, and a fifth wheel on top to support the front of the second trailer. The dolly converts the rear trailer's kingpin (which would normally connect to a tractor's fifth wheel) into a tow-able unit. Coupling a double: pull tractor in front of first trailer, couple normally; tow the dolly to position behind first trailer; couple the dolly to the first trailer's pintle hook; back the second trailer onto the dolly's fifth wheel; connect air lines and electrical between all units; check all connections.