Ball boys and girls are not meant to be noticed. Neither are members of another tennis tournament crew. But they are essential for making the event run smoothly. They are the line umpires. Up to nine of them stand on the court for each big match.

Line umpires have one job—deciding whether the ball is in or out. They have to stick to their decisions even while the crowd roars disagreement. Spectators might say they have the best seats in the house. But the line judges have no time to notice.

“When you’re down on the court, you can feel the tension and all the crowd reaction,” veteran judge Andy Davies tells The Times of London. “You’ve got to stay focused.”

A chair umpire monitors the full set of line judges for every match. (As many as nine are needed in a pro match!) Are their calls clear? Are they accurate? Do they do well under pressure?

Chief Wimbledon umpire Adrian Wilson says, “It’s about showing confidence that you’ve got the call right.”

Every call a line judge makes might be challenged. Viewers and TV commentators have their own ideas about where the ball landed. And many players feel that they know by instinct whether a ball is in or out.

Now some major tennis matches are also tracked by “Hawk-Eye.” This uses six or more computer-linked television cameras around the court. Each computer reads the video. It tracks the path of the tennis ball on each camera. The views are combined to produce an accurate 3D picture of the path of the ball.

This puts more pressure on line umpires to be accurate. According to The Times, Hawk-Eye confirms that line judges’ challenged calls are correct nearly three-fourths of the time.

How well would you do? Ask permission to play a game to make your own calls at www.thetimes.co.uk/tennislinejudge.

Andy Davies says, “I hate making mistakes. But it feels good when you get it right.”

Prove me, O Lord, and try me; test my heart and my mind. — Psalm 26:2