I purchased 2 of these collars for my two dogs; the dogs are about 35-40 pounds each. These collars do not stay closed. Several times my dogs have pulled and the collars have simply fallen off. The issue is that the prongs are made of weak metal, so they separate easily. It just happened again today during a walk along our street. One of my dogs took off in an unfamiliar area (to the dog) and was narrowly missed by passing cars. These are going into the trash to be replaced with stronger metal prong collars.