Click on the question to reveal the answer below:

Q1: What are the court regulations for international competitions with respect to the ATP, WTA and ITF Davis Cup? (answer opens in new window)

Q2: What is the difference between a pressurised and a pressureless tennis ball?

Q3: Where can I find a list of approved balls and classified court surfaces?

Q4: What is the process for obtaining tennis ball approval?

Q5: What is the process for obtaining classification of a tennis court surface?

Q6: How do I build a tennis court?

Q7: I have devised a new piece of tennis equipment. How do I get it approved?

Q8: How do rackets with piezoelectric fibres work?

Q9: What is the difference between Coefficient of Restitution and Apparent Coefficient of Restitution?

Q10: I have read research studies in which rackets have been firmly fixed in place and others in which they have been freely suspended. What is the difference between these two conditions?

Q11: What does it mean when a tennis ball has “set”?

Q12: I want to set up a tennis court, but don’t know which way it should be oriented?

Q13: How many tennis balls are made each year?

Q14: How many tennis courts are there world-wide?

Q15: How long is a tennis net?

Q16: What are the main technological advancements that have taken place within the game of tennis over the past 25 years?

A2: There are two basic types of ball, which are differentiated by their internal pressures.

The pressurised ball may be thought of as the traditional type and accounts for the majority of tennis balls manufactured throughout the world. The hardness and resilience properties of the pressurised type result from both the modulus of rubber in the core and the internal pressure of the core itself. Although this construction gives a preferred type of ball in terms of playing properties, it has the disadvantage of restricted shelf life once it has been removed from its pressurised container. This is because the pressurised gas in the core diffuses through the rubber wall over time leading to a gradual reduction in ball hardness and resilience.

The pressureless ball is designed in such a way that the required hardness and resilience is achieved solely by the rubber core and the internal pressure is equal to atmospheric pressure. Consequently there is no pressure loss from a pressureless ball and no change in properties attributable to changes in pressure. Neither is it necessary to pack pressureless balls in pressurised containers.

The scientific explanation for the observed difference in playing characteristics of the two types is the difference in elasticity. The pressureless ball is less elastic and so experiences a higher hysteresis loss than the pressurised type when subjected to deformation which means that more of the impact energy is lost during contact with the racket and there is thus less returned so imparting less ball speed.

For more information on tennis balls and their manufacture please refer to the tennis balls equipment page.

A3: The ITF publishes a booklet in January each year.

An on-line version can be downloaded from the publications section by clicking here.

Alternatively, a hard copy can be obtained by e-mailing technical@itftennis.com;

Or by post to: ITF Technical Centre Bank Lane Roehampton London SW15 5XZ United Kingdom

A4: To apply for approval of a tennis ball, please contact the ITF Technical Centre.

e-mail: technical@itftennis.com

Post: ITF Technical Centre Bank Lane Roehampton London SW15 5XZ United Kingdom

An on-line version of the ITF Approved Tennis Balls and Classified Court Surfaces, which contains the relevant rules of tennis, regulations for making tests and the testing procedures, can be found by clicking here.

A5: To apply for classification of a tennis court surface, please contact the ITF Technical Centre.

e-mail: technical@itftennis.com

Post: ITF Technical Centre Bank Lane Roehampton London SW15 5XZ United Kingdom

An on-line version of the ITF Approved Tennis Balls and Classified Court Surfaces, which contains the procedures and guidelines for surface pace testing, can be found by clicking here.

A6: The ITF Technical Centre is currently gathering information that, when complete, will be posted here.

There are some guidelines on court construction currently avaliable here and a short description of asphalt court constrcution is detailed here.

More information can also be found via the two organisations below:

Tennis court builder’s association UK

Tennis court and track builder’s association USA

A7: Approval currently only applies to tennis balls.

If you have produced a ball that you intend to be used for play according to the Rules of Tennis, then it must be approved by the ITF Technical Centre (see contact details below).

If you have produced any other item of tennis equipment (such as a racket, vibration damper, automated line-calling system etc.) and wish to have it evaluated by the ITF to find out if it conforms to the Rules of Tennis, please contact the Technical Centre:

e-mail: technical@itftennis.com

Post: ITF Technical Centre Bank Lane Roehampton London SW15 5XZ United Kingdom

A8: Piezoelectric material is a ceramic that is composed of dipoles (the ends of each molecule have opposite charges), and in which the dipoles have all been oriented in the same manner (polarised). This results in a material that is electrically neutral in its resting state, but has the potential to create a current in an electrode.

Piezoelectric materials require energy to operate, and this is derived from the energy transferred from the ball to the racket during impact (energy sources are not allowed to be built into, or attached to, a racket). When the piezoelectric material is stretched or bent – as happens when a ball hits the racket – the displacement from the dipoles’ resting positions increases their electric charge and creates a current through the electrodes.

The electrical current produced during impact activates a microchip, which returns it back to the electrodes after a set interval. This current causes the piezoelectric fibres to be activated in direction opposite to that in which they were stretched, and so tends to cancel vibrations.

Stretching the fibres in the opposite direction starts the same process again, which continues until damping is complete.

A9: The performance (or ‘power’) of a tennis racket is expressed in terms of the speed of the ball that is produced after impact in terms of the relative velocities of the racket and ball immediately before and after impact.

Apparent Coefficient of Restitution (ACOR) is simply the velocity of the ball after impact divided by the relative velocity of the ball and racket before impact:

ACOR = Post-impact ball velocity ÷ Pre-impact relative velocity

Coefficient of Restitution (COR) takes the effects of impact on the racket into account:

COR = Post-impact relative velocity ÷ Pre-impact relative velocity

If we consider impact between a ball and a stationary object (such as a floor), then COR and ACOR will be equal, as the floor is static before and after impact. During impact between a ball and racket, however, both change velocity.

A10: In some instances, the answer is that there isn’t a difference.

When a ball hits a racket, the impact duration is approximately 5 milliseconds (0.005 s). In such a short time, the way in which a racket is ‘gripped’ has no effect on the ball, because the vibrations generated by the ball’s kinetic energy don’t have sufficient time to get to the butt of the racket and back to the head.

It should be noted, however, that this only applies to impacts on the long axis of the racket (i.e. a line along the handle extended into the racket head). When the ball hits the racket head somewhere else, it tends to make the racket twist about the long axis. If a racket is firmly clamped, the twist will be resisted and the ball will rebound differently compared to a racket which is freely suspended.

A11: When a tennis ball is made and packaged, it is not subject to large external forces as would be experienced during play.

Thus, although the rubber core is an elastic material, it tends to become more resistant over time to changing its shape and, consequently, behaves differently when compressed. The original characteristics return after a short period of compression, which is why the first process in the approval testing process is to compress each ball by 2.54 cm (1 inch) on 3 perpendicular axes a total of 9 times.

A12: Courts should be positioned so that the centre line runs in a north–south direction.

This is to minimise distraction to players and spectators when the sun is lowest in the sky (which tends to be either in the east or west). In some latitudes a small deviation from true north may be preferable due to solar orientation during the playing season.

A13: It is estimated that around 30 million dozen (360 million) tennis balls are manufactured each year.

Laid side by side this many balls would form a line measuring 23,400 kilometers in length – almost enough to stretch from London to New York four times over. This number of balls would form a single stack weighing almost 21,000 metric tons!

A14: It is conservatively estimated that there are 750,000 tennis courts in the world (but there could be as many as 1 million).

Joined together this number of courts would form an area measuring almost 194 square miles – larger than some small countries!

The lines painted on all these tennis courts would, if combined, form a line measuring over 68,000 miles long or a quarter of the distance to the moon.

A15: The net on a tennis court is 42 feet long (12.8 m).

Many people consider that the game of tennis has changed significantly over the past 20 - 25 years and has become faster and far more ‘power’ orientated than in the 1960s and early 1970s when skill and ‘touch’ players competed with those who relied on a big serve. Although it is a common perception that developments in equipment that have led to this situation, this cannot be entirely justified. Attributing changes in tennis as a whole to equipment is difficult – and impossible to quantify – because performance is not always directly measured (as it is in, say, javelin throwing). Furthermore, development of the athlete has occurred in parallel with equipment modification. However, by examining each factor in isolation, it is sometimes possible to estimate how it would influence tennis if all else remained constant.

The ball

Certain aspects of the game have hardly changed. The ball, for example, has been made to a relatively tight specification, as laid down in the Rules of Tennis, for many years. However, following a two-year experimental period, from 1 January 2002 the Rules of Tennis (Rule 3) have been amended to permitt two new types of ball – a fast speed ball (Type 1) and a slow speed ball (Type 3). The two new types of ball are designed to have specifications that will result in different performance characteristics derived from their differing dynamic and aerodynamic properties.

A16: The game in general.

- Type 1 (fast speed) is identical to the standard ball except that it is harder, faster and is designed for use on slow surfaces such as clay.

- Type 2 (medium speed) is the standard ball and is identical to the specification that existed prior to 2000.

- Type 3 (slow speed) is identical to the standard ball but is approximately 6% larger in diameter.

Research has indicated that the larger ball possesses performance characteristics that differ from the standard ball in two ways:

1) A larger diameter ball possesses different aerodynamic drag characteristics to those of a conventional size ball. Such a ball will therefore slow down more quickly during its flight and give the receiver more time to react.2) A second characteristic of the larger ball is one that is related to the rebound angle of the ball after impact with the court surface. This is partly due to its steeper incoming trajectory and partly to the different compression characteristics during impact. The rebound angle of this ball has been found to be higher than that of an existing ball, again giving the receiver marginally more time to react and return the ball.

Both of the above characteristics combine to produce a ball type that is ideally suited to a court surface type that is considered to be ’fast’. Surfaces that fall into this category would be grass, some artificial turf and indoor carpets, wood, etc.

The larger ball is also suitable for the recreational game on all surfaces. For recreational players, who typically hit the ball at lower speeds, the increase in reaction time is even greater, allowing more time for stroke development and better control of the ball.

So the Type 3 (larger, slower) ball is designed to address both the difficulty that some recreational players have in learning to play tennis, and the perceived problem with the growing dominance of the serve in the professional game.

The racket

Few sports products can have evolved as quickly and as far as tennis rackets have over the last 25 years. Rackets are now considerably lighter and stiffer, and feature larger head sizes than 25 years ago. Modern rackets have larger 'sweet spots' and are generally more 'powerful' and so are easier to use particularly at the recreational level.

The player

Male and female professional players are now taller, fitter and stronger than players of a generation ago, although it is difficult to quantify the contribution that this evolution has contributed to the changing nature of the game. There is no doubt that they hit the ball harder and move faster around the court, and are likely to continue to do so.

The role of the ITF Technical Centre

The ITF, as governing body of the sport, is committed to a policy of studying all aspects of the game, so that we are in a position to make the changes to the regulations as necessary. We have invested heavily in our own in-house Technical Centre, a laboratory which enables us to study aspects of court surfaces, balls, rackets etc. and the various interaction between them.