Hybrid Guide

A hybrid (aka blend) string installation involves the use of two different strings for the mains and crosses.  Generally, main strings break first as they tend to move more during impact and therefore endure more abrasion than the crosses.  As a consequence, usually a hybrid installation will assign a more durable (harder) string to the mains and a more playable (softer) string to the crosses.  Durability may not be the primary motive for choosing a hybrid though, often hybrids are chosen partially or entirely for their performance characteristics.  The chart below offers some insight into the performance and durability characteristics of possible hybrid combinations. 

Terminology:

Cross Strings:

Strings running perpendicular to handle axis.

Main Strings:

Strings running parallel to handle axis.

Solid-Core nylon based examples:

Golden Set Synthetic Gut, Classic & Optimum (more info)

Multi-Filament examples:

Golden Set Velvet & Maximal (more info)

Aramid (aka Kevlar) examples:

Golden Set Hard-Line (more info)

Co-Polyester Examples:

Golden Set Poly, Hex Poly & Snake-Bite (more info)

#1
MAIN STRINGS: Co-Polyester
CROSS STRINGS: Solid-Core Nylon Based
SUMMARY: These are the most popular hybrid combinations. The co-polyesters’ durability, spin generation and controlled power dominate while the soft, nylon, solid-core crosses provide a noticeably more forgiving feel.
GAUGE CONSIDERATIONS: Similar main and cross gauges generally perform best.

#2
MAINS: Aramid
CROSSES: Co-Polyester
SUMMARY: Because Hard-Line is composed of aramid fibres (eg: Technora/Kevlar), it is an extremely stiff string. It should almost always be paired with a soft cross string to compensate for this stiffness. Co-polyesters are not a good cross string choice because they are also stiff.

#3
MAINS: Aramid
CROSSES: Solid-Core Nylon Based
SUMMARY: When installed as the main string, Hard-Line is most commonly paired with a solid-core nylon based cross string. Solid-core nylon based strings are soft enough to compensate for the extreme stiffness created by Hard-Line’s aramid fibres. 
However, durability is not the only reason for installing aramid. Some players actually prefer the stiff feel of aramid over the more forgiving nature of other string types.

The shorter contact time also increases the peak impact force leading to the firmer (eg: “board-like”) feel associated with aramids. This firmer feel is less than desirable for players who experience tennis elbow and other shock related sensitivities.
Thanks to aramid’s superior durability, extremely thin gauges can offer adequate durability. Hard-Line 19 (diameter = 1.10mm) is a good example of this. Due to its extremely thin diameter, this string offers substantially more spin potential and elasticity than thicker aramid gauges.
GAUGE CONSIDERATIONS: Generally the aramid main string needs to be a similar or thinner gauge than the cross string.  

#4
MAINS: Aramid
CROSSES: Multi-Filament
SUMMARY: Due to aramid’s extreme stiffness, the soft and forgiving characteristics of the multi-filament crosses will only be marginally apparent. The durability of the multi-filament crosses will be a concern. Solid-core nylon based cross strings are less expensive than multi-filaments while offering superior durability and a surprisingly similar feel in this particular role.

#5
MAINS: Co-Polyester with Textured/Rough Surface (eg: Golden Set Hex Poly or Snake-Bite)
CROSSES: Co-Polyester with Non-Textured/Smooth Surface (eg: Golden Set Poly)
SUMMARY: Installing a smooth co-polyester in the crosses with a textured co-polyester in the mains offers increased durability as compared to using a textured string in both the mains and crosses. The texture increases spin generation while the smooth crosses reduce wear of the mains to improve durability.
GAUGE CONSIDERATIONS: Similar main and cross gauges are most common.
In cases where gauges are dissimilar, the crosses ought to be thinner since the mains endure more localised stress and abrasion than the crosses. A setup such as this yields increased spin potential and elasticity.
However, a thinner cross string leads to a moderate loss of durability. A smaller diameter has a smaller, sharper radius of curvature that reduces the contact area between the mains and crosses. This reduction in contact area results in higher contact pressures than if the crosses were the same thickness as the mains. Higher contact pressure equals more abrasion and therefore reduced main string durability.

#6
MAINS: Multi-Filament
CROSSES: Co-Polyester With Non-Textured/Smooth Surface (eg: Golden Set Poly)
SUMMARY: This combination provides a softer feel with excellent spin potential and surprising durability. While such a combination will generally not last as long as the reverse (eg: smooth co-poly in mains and multi-filament in crosses), it will provide a softer feel.
GAUGE CONSIDERATIONS: Similar gauges are generally used in the mains and crosses.

#7
MAINS: Multi-Filament
CROSSES: Co-Polyester with Textured/Rough Surface (eg: Golden Set Hex Poly or Snake-Bite)
SUMMARY: This combination is uncommon due to durability concerns. The softer multi-filaments tend to abrade rather quickly when paired with the much harder, rough textured co-polyesters.

#8
MAINS: Solid-Core Nylon Based
CROSSES: Multi-Filament
SUMMARY: A very cost-effective and durable combination with a soft, arm-friendly feel. The less expensive solid-core mains deliver good durability while the multi-filament crosses provide a comfortable feel.
GAUGE CONSIDERATIONS: Similar gauges are generally used in the mains and crosses.

#9
MAINS: Co-Polyester
CROSSES: Multi-Filament
SUMMARY: The co-polyesters’ durability, excellent spin generation and controlled power dominate. The very soft multi-filament crosses effectively moderate the sometimes undesirable co-polyesters’ stiffness. This “softening” effect is more pronounced than what can be obtained by using a nylon based solid-core string in the crosses.  On the other hand, the multi-filament crosses’ lower durability may be a concern. A smooth co-polyester (eg: Golden Set Poly) should be used in the mains if greater durability is desired.
GAUGE CONSIDERATIONS: The multi-filament crosses are generally of a similar or thicker gauge than the co-polyester mains.