Disk-parking is a system of allowing time-restricted free parking through display of a parking-disk or clock-disc showing the time at which the vehicle was parked. A patrolling parking attendant can inspect the disk to check whether payment is owed. The system is common in Europe. The original system had been introduced along with establishing a Blue Zone area. The blue zones in the city-centers allow for free parking of about 2 hours by using a parking-disk issued by the city while other parking zones may have stricter control by parking meters. The current unified EU parking-disk-regulations allow for a wider range of time restrictions that are indicated on the signs. Procedure Upon arrival at a parking lot requiring parking disc, the parking disc is set and put on display behind the front windshield. The disk has a thumb wheel (or knurling-wheel) at the upper side that makes the clock face turn around its center point, allowing it to show different clock times. The standard parking clock can show a full twelve hours, with additional marks for each half-hour. The time to be set is rounded up from the actual arrival time, for example a 10:00:01 arrival would be set to 10:30, while a 10:29:59 arrival would also be set to 10:30. The legal requirements are that "parking is allowed when the vehicle shows a parking-disk that is easily readable from the outside and whose clock-hand points to the mark of the half hour that follows the time of arrival". The latest departure-time is calculated from the displayed arrival-time on the parking-disk based on the indicated maximum parking-time. For example, if the parking-disk was set to 10:30 and the parking-sign indicated a one-hour maximum parking time, then free parking would end at 11:30. The system is intentionally set up in a way that only full and half hours are used to indicate arrival and departure. Arriving at 10:02 in a parking disc zone of one hour maximum parking-time invokes a departure time of 11:30, and hence an effective time interval of eighty-eight minutes for free parking after arrival. In short, if the parking-sign shows a limit of one hour maximum parking-time, then the effective maximum parking-time is sixty-one to ninety-minutes because the difference to the next half hour is added. Drivers risk getting a ticket (if a parking enforcement-officer arrives before the beginning of the indicated interval) if they set the