Because the line holding up the net forms a curve. The Earth's gravity pulls it down in the middle. It is held up at both sides. Check out washing lines, electric power lines, telephone lines: they all sag in the middle.
The net should be 3' (36") or 91.5cm at the center strap, and 3'6" (42") or 106.5cm at the post. You can stand your racquet on the ground for reference.
A table tennis net is 6 inches high at the center. It is slightly higher at the ends, where it is attached to the posts, but the official height measurement is taken at the center of the net.
The official height of a tennis court net is three feet in the middle of the net. At the posts, located on each side of the court, the net should be three and a half feet high.
The net's height is 5' at the center and 5'1" at the poles.
In tennis, the net is stretched across the full width of the court. For single play, this is 27 feet. Doubles requires a 36 foot net.
A standard tennis court for singles matches measures 23.77 meters in length and 8.23 meters in width. For doubles matches, the width increases to 10.97 meters, while the length remains the same. The service boxes are located 6.40 meters from the net, and the net height is 0.914 meters at the center.
The standard tennis net is three feet high, exactly one yard.
The tennis courts are 23.78 meters long and 10.97 meters wide. Tennis court terrains are grass, clay, hard, carpet and indoor. There is a low net in the center of the court.
sINGLES STICKS ALLOW THE NET TO BE AT AN APPROPRIATE HEIGHT RIGHT THE WAY ACROSS THE NET WHEN PLAYING ON A COMBINED SINGLES/DOUBLES COURT.
The official height of a table tennis net is 15.25 cm(6.0 in). Which converts to 0.1525 metre.
The tennis service line is located 21 feet from the net on a tennis court.
The height of the net is 1 m height and the height of the racquets may vary according to the model you buy.