Polyester strings. Stiffer and more durable than gut or synthetic strings. Poly generates more spin because the strings snap back after making contact with the ball, gripping and releasing it. The tradeoff is less feel and more arm stress. The polyester string revolution in the early 2000s is partly why modern tennis has so much topspin. Nadal, Djokovic, and most current pros use full poly or poly hybrids.
"Full bed of poly. She loves the spin but re-strings every week because poly goes dead fast."