Superglide glass feet are designed to improve the gliding performance of your mouse, but they do not come with the middle ring that surrounds some mouse model's sensors. The reason for this is because glass is a reflective surface that can interfere with the sensor's ability to accurately track movement. Therefore, to ensure that your mouse continues to function properly, it is important to keep the original PTFE ring that was included with your mouse model, or purchase a new one separately if needed. Without the ring, your mouse's sensor may not be able to detect movement accurately, leading to reduced performance and accuracy. So, be sure to keep or replace the ring to get the best performance out of your mouse with Superglide glass feet.