Abstract: An electrically operated line monitor for tennis wherein different characteristics are used for rays which are projected from the same first position to the same second position. In one embodiment of the invention, to monitor the center line (104) on a tennis court, a master ray (111) is projected down the center line (104) and has different characteristics to two other rays (114, 117) projected on either side of the master ray (111) and over the service courts (108, 107) adjacent to the center line (104). In another form of the invention, a ray (128) which can be a master ray is projected just inside a critical line from a first position to a second position, a diverging ray (133) having the same characteristics as the master ray is projected from the second position to the first position and another diverging ray (135) having different characteristics to the two other rays is projected from the first position to the second position; this prevents interference due to careless setting-up.