Early clay courts are thought to have been invented in England where the blazing sun of the summer months would cause grass courts to burn. Clay was then initially used as a protectant, this slowly evolved into the clay courts we enjoy today. Modern clay courts began to appear during the mid-1900s. Ground up brick or locally available clay was typically used, when the first Har-Tru courts were unveiled in the 1930s, the use of clay courts became more standardized and predictable. 

Clay courts are the slowest of the three courts. In the same ITF experiment where they shot a ball without spin at 67 mph, they found that on a clay court the ball slowed down to a sluggish 38 mph. Because the clay court has such a high coefficient of friction, it undergoes an increase in spin where the bottom of the ball slows more than the top. This gives a burst of topspin which transfers some horizontal velocity in a vertical direction which makes the ball bounce straight upwards. Clay courts have a high coefficient of restitution which ends up making the ball rebound at a higher angle than it originally hit the ground at. 
​