I measured just above the top of my knee, which was 19" so I ordered the LG/XL size, but it was too big and the top strap kept sliding down the middle of my kneecap. I should have ordered the SM/MD size. Also, in addition to there be tubes above and below you kneecap, there are tubes on each side of the kneecap - but for me, because the size was too big, the side tubes were no where near the sides of my kneecap. The packaging says to measure around the CENTER of your knee (see attached photo), not above your knee. I also found this too be uncomfortable around the back of my knee.
Rating: [4 of 5 Stars!]