ANOTHER UPDATE: I received the replacement order that I placed a few days ago and the socks have exactly the same problem. The gray socks have silicone in the heel to keep them from slipping but the black socks have no silicone. As I was refunded for my first order and then paid for a new order I have ended up with 4 pairs of socks that work for the $14.99 I paid. However, it would be unfair of me not to report that two of the replacement pairs of socks were not usable due to a manufacturing problem (according to customer service the socks should all have the silicone in the heel). So I am giving two stars for the good customer service but cannot give any more due to the quality control issues with the product. If these had been on prime when I reordered I would return them but they were not so I am unable to send them back.

UPDATE: A few hours after posting my review (below) I was contacted by the seller who explained I must have received an unfinished product. They offered to replace the order by refunding me so I could order them again. They refunded me the same day-- great customer service! I am about to place a new order for the socks and hopefully will not experience any further issues. I am updating my review to 5 stars as the socks that have the silicone strip in the heel work very well and the customer service from this vendor has been excellent!

ORIGINAL REVIEW
This package came with 4 pairs of socks; 2 x gray and 2 x black. The gray ones are as expected. They have a small silicone grip on the inside of the heel that stop the sock from coming off and ending up under your foot. For some strange reason, the black pairs do not have this silicone on the heel so after a few minutes of walking the heel comes off and the sock ends up under your foot. So basically the 2 black pairs are useless and unwearable. I am giving two stars because I could use 2 of the pairs and those were good. By the time I discovered that the black socks were useless I had already worn both of the gray pairs so I was not able to return them. I feel like I got ripped off paying for 4 paris of socks and receiving two duds. If you are willing to throw half the socks in the trash, go ahead and order them. If not, I recommend you find a different seller!