Poly
The broad polyester family
"Poly" is the general bucket. Players use it as shorthand for modern monofilament polyester strings built for control, durability, and fast-swing performance.
Reference Guide
Poly vs Co-Poly
In everyday tennis talk, these two terms are often used almost interchangeably. In practice, most modern performance polyester strings are really co-polys, but the distinction still helps explain why some strings feel firmer, softer, livelier, or more muted than others.
Quick Answer
What the difference usually means
Co-Poly
Poly with added ingredients or blends
Co-poly usually means the string is still polyester-based, but the formula includes additives or blended materials to change feel, stiffness, tension behavior, or snapback.
Real-world use
Most players just mean "poly"
On court and in shops, many players say poly even when the string is technically a co-poly. That is why the terms can sound messy in conversation.
Practical Feel
How they often differ in play
Older/Firmer polys
Usually stiffer and more direct
Traditional polys can feel crisp, firm, and low-powered. They often appeal to players who swing fast and want a strong control baseline.
Softer co-polys
Usually easier to live with
Many co-polys are built to soften the harshness players used to associate with polyester. They can still be control-oriented, but with a more forgiving response.
Not a hard rule
Formula matters more than the label alone
Some co-polys are still very firm, and some strings marketed simply as poly are softer than expected. In the end, specific formulas matter more than the headline label.
Best Use
How to think about the choice
If you want control
Either category can work
The real question is not poly versus co-poly in the abstract. It is whether you want a firmer, crisper response or a slightly softer, friendlier version of that same control style.
If you need comfort
Move toward the softer end
If you are set on polyester, softer co-polys are usually the first place to look. But if comfort is the real priority, you may need to move beyond poly entirely into multi, gut, or hybrid territory.
Simple rule
Use the terms as direction, not law
Treat co-poly as a sign that the string may be trying to refine the classic poly feel. Then judge the actual string by how it performs, not just by the word on the package.