At the start, signalled by a cannon, each crew is separated by a distance of about 1½ boat lengths (approximately 30 m or 90 ft). Once the race has begun, a crew must attempt to catch up with the crew ahead of it and "bump" (physically touch or overtake it) before the crew behind does the same to them. A crew which bumps or is bumped must pull to the side of the river to allow other crews to continue racing. A crew which neither bumps the crew ahead nor is bumped by the crew behind before crossing the finishing post is said to have "rowed over". Any crew which bumps then swaps places with the crew that it bumped in the following day's racing. A crew which rows over stays in the same position. Crews finishing at the top of a division, the "sandwich boat", row at the bottom of the next division to try to move up a division. The process is repeated over four effective days, allowing crews to move up or down in the overall order of boats. The finish order of one year's Lent Bumps is then used as the starting order of the following year's races. Due to the shortness of reliable daylight, the races are actually currently run over five days, with one division level dropped out each day except Saturday: on Tuesday M/W 1, on Wednesday 2, Thursday 3 and Friday 4.