Introduction to Stringing Theory & Jargon

To most players racquet strings are usually an afterthought and they will probably remain that way. A typical club player thinks nothing of spending between £30 and £100 on a racquet and ironically usually hits the ball with the strings which are likely to have been unconsidered in the purchase and do little justice to the quality of the design, construction and playing capability of the racquet. This is particularly true for brand new racquets (those bought in the UK usually come pre-strung) often strung in the far east using cheap strings chosen for their ability to tolerate the very fast stringing techniques used in their stringing process rather than the requirements of the player who has paid considerably for a high performance racquet. They have often been strung too tightly for optimum play and then stored for some time during which they have lost elaticity and become "dead".
 

In my descriptions elsewhere on this site I use the same terms (jargon – sorry!) as used by manufacturers and professionals in the racquet stringing industry to describe the properties of string and offer here an explanation of those terms in order to reduce misunderstanding and ambiguity and hopefully even add some clarity. I believe the terms used are valid in describing the properties of racquet string even if they're often hyped up when considering the benefit of one string over another. Truthfully a little practice is likely to make a bigger difference than the string used by the average player...
Comfort:  The ability of the string-bed to absorb the shock from the impact with the ball so as not to induce any repetitive strain injury such as tennis elbow.
Control:  The ability to send the ball in the desired direction, to the desired location and to impart spin as intended. 
Cross strings or “crosses”:  The strings running orthogonal to the shaft of the racquet and to the main strings.  They cross the main strings and are ususally strung with less tension than the mains as there are more of them.
Creep:  Change in the physical property of a string material under tension in which the molecules resettle in a different position with respect to each other and stop trying to return to their original positions. This is a loss of elasticity with a consequent loss of string tension and resilience. Creep explains why strings feel dead after they have been in a racquet too long.
Durability:  How long a string lasts before it either breaks through wear and tear or impact or it becomes “dead” through lack of elasticity.
Dynamic stiffness:  The opposite of “pocket elasticity” which is easier to understand. The stiffness referred to here is longitudinal – along the length of the string – rather than lateral. The degree to which a string will resist stretching locally to the force as more force is applied to it.  A dynamically stiff string will stretch evenly across its length rather than more in the vicinity of where the force is being applied by the ball. Low dynamic stiffness leads to “pocket elasticity” which leads to increased power.
Feel:  The degree to which the player can feel the contact of the ball with the strings.  This will depend on the texture of the string, which affects how it grips the ball on impact, and the time for which the strings are in contact with the ball which is influenced by the elasticity, stiffness and tension of the strings.
Elasticity:  The amount a string will stretch under a given force.  A more elastic string will stretch further.  The elasticity of a string will be different at different tensions.  String manufacturers also talk of: “Dynamic elasticity” or (also referred to as "Pocket Elasticty") meaning the string returns back to its original length more quickly when the load causing the stretch is removed. This is likely to be hyped but theoretically imparts more force on the ball providing more power and will be higher at higher string tensions.
Gauge:  The thickness of the string.
Hybrid or "hybrid string pattern":  It is becoming common to use a stronger string in the main strings and a softer string in the crosses to enable a better combination of durability and feel. This is known as a hybrid string pattern. As the mains generally wear out first anyway this can be effective for anyone who wants to improve the feel of their strings by sticking with the main string they normally use and switching to narrower gauge on the crosses or even a softer cross string altogether.
Main strings or “mains”:  Those strings running parallel to the shaft of the racquet or closer to parallel with the racquet shaft in some of the modern fan shaped string patterns. They are ususally strung with more tension than the corsses as there are less of them.
Pocket elasticity: meaning the elasticity localised in the immediate vicinity of the ball and hugging the ball rather than across the whole string-bed. For shots played "off-the-string-bed-centre" the “trampoline effect” from the recoil of “pocket elasticity” tends to be less skewed than for the overall string-bed "trampoline effect" leading to less loss of control. “Pocket elasticity” comes about when a string has lower “dynamic stiffness”.
Tension:  The amount of stretching force within the string. A certain tension threshold is required before the required elastic properties of the string come into play. This threshold differs for different string materials, constructions and gauges.