Considerable difficulty is often experienced by beginners in marking out a court, and, in fact, it is not a simple matter. The first thing of importance is to determine generally one corner of the court and to get a base line and a side line at a true right angle of ninety degrees. The same principle may be employed that is used by builders and surveyors in “squaring a building,” as it is called. You will need a ten-foot pole with marks for the feet indicated on it in lead pencil, and in addition to this a few 20-penny spikes and a ball of stout twine. Drive a nail into the ground where you want one corner of the court and fasten the line to it; then stretch the line to another nail to mark either a side line or back line. You will then have one side and the corner fixed, and the problem is to get another line at right angles to it. Boys who have studied geometry know that “in a right-angle triangle the square of the hypotenuse is equal to the sum of the squares of the other two sides.” It isn’t necessary to understand this, but it is the principle employed in “squaring.” You next stretch another line and have some one hold it. On the fixed side line you measure eight feet from the corner nail and mark it with a piece of twine tied around the line. You also make a six-foot mark on the line to be at right angles to it, the exact direction of which is yet to be determined. Both of these measurements must be accurate. The boy on the end of the loose line moves it until the distance between the two pieces of twine is exactly the length of your ten-foot pole. The angle thus formed is exactly ninety degrees, or a right angle. Having obtained one side and one end, to finish marking is simply a matter of making the necessary measurements of a court as shown on the diagram and marking each intersecting point with a nail driven into the ground. [Illustration: How to mark out a tennis court] Another way to lay out a court is to drive two stakes or nails into the ground 27 feet apart. (The line of these stakes should be the position of the net.) Then take two pieces of twine, one 47 feet 5 inches long, and the other 39 feet. Fasten one line to each of the spikes that you have placed 27 feet apart. Where the two lines meet as they are pulled taut are the true corners of the court, as there are only four points where they can meet. The various measurements can then be marked as above by referring to the diagram. It is customary to mark a double court and to indicate the lines for singles afterward. The game of tennis may be played either by two or four persons, or sometimes an expert player will stand two beginners. The ball used is rubber filled with air and covered with white felt and is 2½ inches in diameter. It is necessary to play with two balls, and to save time in chasing those that go wild it is customary to play with three or four. [Illustration: Photographs of Tennis Strokes Taken in Actual Play]