Tennis is a ball designed for tennis. Tennis is fluorescent yellow in organized play, but can be almost any color in casual play. The tennis balls are covered with fiber felt, which changes their aerodynamic properties, and each tennis ball is covered with a white curved oval.
Modern tennis balls must meet certain size, weight, deformation and bounce standards in order to be approved for regulated play. The International Tennis Federation (ITF) defines the official diameter as 6.54–6.86 cm (2.57–2.70 in). The mass of the ball must be in the range of 56.0–59.4 grams (1.98–2.10 ounces). Yellow and white are the only colors approved by the ITF, and most balls produced are a fluorescent yellow known as “optical yellow,” first introduced in 1972 after research showed they were more visible on television.
The tennis ball is filled with air, and the surface is covered with a uniform layer of felt-rubber compound. Felt delays airflow separation in the boundary layer, which reduces air resistance and gives the ball better flight performance.  In addition to the brand name, there is usually a number on the ball. This helps distinguish one set of balls from another set of the same brand on an adjacent court.
As soon as the tennis can is opened, the tennis ball begins to lose its elasticity. They can be tested to determine their rebound. Modern rules tennis balls are kept under pressure (about two atmospheres) until initial use; balls used at high altitudes have lower initial pressures, and inexpensive practice balls are made without internal pressurization . Test ball bounce by dropping it onto concrete from a height of 254 cm (100 in); 135 to 147 cm (if occurring at sea level and 20 °C (68 °F) and 60% relative humidity Bouncing between 53 and 58 inches) is acceptable; high altitude balls have different characteristics when tested at sea level.