Some car washes have their customers pay through a computerized "POS", or point of sale unit, also known as an "automatic cashier", which may take the place of a human cashier. The mechanism inputs the wash PLU into a master computer or a tunnel controller automatically. When the sale is automated, after paying the car is put into a line-up called the stack or queue. The stack moves sequentially, so the wash knows what each car purchased. After pulling up to the tunnel entrance, an attendant usually guides the customer onto the conveyor. At some washes, the system will send the correct number of rollers automatically, based on tire sensors. The tire sensor lets the wash know where the wheels are and how far apart they are. On other systems the employee may guide the customer on and press a 'Send Car' button on the tunnel controller, to manually send the rollers which push the car through.