W WHITE LINES - they mark the boundaries and zones of a tennis court. In the modern Grand Slam tournaments they are monitored, in relation to where the ball lands, with the aid of the "Hawkeye" computer system, which was first officially used in the US Open in 2006. Before that it was just the decision of the line judges, and ultimately, the umpire. (See previous entry for "U").
There was a 1982 novelty Wimbledon UK hit single "Chalkdust - the Umpire strikes back" by "The Brat" (Comedians Roger Kitter and Kaplan Kaye) . The song was loosely based on the antics of US tennis star John McEnroe, in relation to disputed line calls pre-"Hawkeye" technology. (Clip on YouTube).