I bought two of these because I dislike having more than one spare mag on my hip since it causes too much bulge. One is for my left leg and one for my right. They fit PERFECTLY snug for my mags (17rd for SIG P226) and are very comfortable and low profile when worn over socks. Even with one mag in the pouch there is still room for another whole mag or a flashlight. I only keep one mag in each though for a total of 4 mags on my body. Best $30 I ever spent for a mag carrier. Only requirement is that pants be worn. They would be even more discreet with single stack mags.
For anyone who doesn't wish to spend money on this you could also wear two socks and slip a mag in between the two so that it fits right between your inner ankle. I did this for a few months and it worked fine. I just got tired of having to wear two socks so I found these.