The basic structure of the spacecraft is called the "bus," which carries the various engineering subsystems and scientific instruments. It is like a large ten-sided box, which can be seen in the Voyager diagram. The centerline of the bus is called the z-axis (and thus the High Gain Antenna) points to Earth. The spacecraft is designed to roll about this axis by firing small thrusters which are attached to the bus.
Each of the ten sides of the bus contains a compartment (a bay) that houses various electronic assemblies. Bay 1, for example, contains the radio transmitters. The bay are numbered 1 to 10 (numbered clockwise as seen from Earth).