In the early days of surround sound, the surround speakers were most often bipoles or dipoles, with two sets of drivers facing in roughly opposite directions. In bipoles, the two sets of drivers are in phase with each other, while in dipoles, they are out of phase, which creates a null region between the two sets of drivers. The goal with both designs is to create a diffuse soundfield with much less localization in the surround channels, which matched the diffuse nature of the mix in those channels.
These days, the surround channels often carry much more localized information, which is why monopoles, also called direct radiators, are becoming more popular as surround speakers. This type of speaker emits sound in only one direction rather than two, allowing it to present a more precise surround soundstage.
Which type of surround speaker do you prefer, and why?