"Clear-Com" or more accurately full duplex, party line, intercom systems. "Full duplex" means that you can talk and hear at the same time (Cell phones are "half duplex" meaning you can either talk or listen). "Party line" means everyone on the same channel is heard by everyone else on that channel. Clear-Com has dominated the market and many people call all intercom systems "clear-com" even if they are made by other manufacturers. Systems consist of a base station, belt packs, and an input device. Input options include table top or wall stations, head sets, CB radio microphones, and telephone handsets. They come in both wired and wireless versions. Wired systems are connected with standard XLR mic cables. You can have multiple channels of intercom so that different departments can talk with each other without disturbing other departments. Manufacturers of intercom equipment include: Clear-Com the traditional king of the market Telex another large manufacturer (football coaches seem to like Telex). Includes RTS, the leader in the TV industry. and Production Intercom A smaller company and personal favorite of some CB users (Check out the BP.15 belt pack). A wise shopper should compare all three product lines when investing in an intercom system.