12m (39') set of Still In Black X10 tennis string in 1.25mm (17 gauge), blue color.

X10 is part of Still In Black's Dual Dynamics line of monofilament polyamide strings designed for use either in a hybrid set or in the full string bed. 

Polyamide is much more flexible than polyester, so it will add power and comfort to a hybrid, or will play comparably to many solid-core or multifilament strings, but with added durability thanks to the large central core and extra-durable outer skin.

Still In Black underwent a 4-year research and development project to develop this line, and the results have been sensational. Players have voiced rave reviews for the performance effects of the X10 string: 

improved power;

lower dwell time for added control;

great touch and feel;

lower effort and reduced injuries

The large, high-elongation core absorbs vibration and dampens impact.

The outer skin stores the energy as it shortens on impact, then returning the energy to the ball.

Thinner 1.25mm diameter for extra feel and spin potential

X10 is intended for higher-level players with good technique and strong shots.