According to human acupoint design 39pcs acupoint massage balls for each foot. What is the Benefit of our Massage Slipper? 1. Promote blood circulation and metabolism 2. Release aches pain such as headaches, migraines, neck pain, lower and upper backaches. 3. Get rid of body's wastes and toxins 4. Relax nerve tension, improve sleep quality What is the difference of our massage slipper with others? 1. our massage slipper using rotating massage head, which is 360 degree rotating massage. NOT UP AND DOWN ONLY 2. our massage slipper added 2pcs jade stone, 2pcs tourmaline stone for health therapy 3. our massage slipper used environmental materials (no heavy smell). ROHS certificate approved Size Reference: S- 10.04inch long(Men's size 6-8, Women's size 7-9.5) M-10.826inch long(Men8.5-10.5, Women10-12) L- 11.614nch long(Men11-13)If you are between sizes, take larger oneP.S:This is acupressure massage slipper, if you are looking for a comfortable all day wear slipper, please DON'T buy it. You may feel painful for first several days, wearing socks and shorten the wearing time will help you feel better. Gradually increase the wearing time after you get used to this foot massage, you will get the benefit soon.