When you use a socket wrench, you have at your disposal a wide range of sockets, each made to fit a particular size bolt, valve or plug. They are made in both metric (millimeters) and standard (inches) sizes. A basic comprehensive socket set includes both metric and standard varieties of sockets ranging from about 6 to 19mm and from roughly 5/32 to over 1 inch. That is not where the differences cease with a socket set. The sockets themselves are manufactured to have a certain number of points on the inside designed to grab on to a bolt and lock the socket into place. Commonly, 6 and 12-point sockets are used. A 6-point socket features six meeting points between interior sides of the socket, while a 12-point socket has twice as many meeting points. Consequently, a 12-point socket looks more like a circle, whereas a 6-point socket is hexagonally shaped.
6-Point vs. 12-Point Sockets
Both sizes have their respective benefits, although some who frequently use sockets may swear by one or the other. Generally speaking, a 6-point socket gives the user less room for error. That is to say that due to the restrictive shape of the socket's interior, only the correctly-sized socket will work for a particular bolt. The benefit of this is that the right socket fits snugly onto the bolt. There is no danger of rounding the bolt's head with a socket.
On the other hand, a 12-point socket more closely resembles a circle on the interior, so it is easier to fit a socket onto a bolt. It's not so much that it's easier, it's that there is more wiggle room. A 12-point socket has twice as many interior points on which it can fit around the 6 corners of a bolt head. What this means in a practical sense is it is possible to use a 12-point socket that is not exactly the right size on a bolt. A good example of this is using a 12-point metric socket on a standard bolt for which you do not have the correct fitting. The 12-point socket will give you some leeway and allow you to loosen or tighten the bolt. The disadvantage of this is that there is a danger of rounding the bolt head, something that cannot happen when you use the right 6-point socket.
Rounding the Bolt
If a bolt head is rounded by an inappropriate socket, it becomes harder to remove because the interior points of the socket no longer have anything to grab onto. For this reason, mechanics usually prefer 6-point sockets. Even though you have to use the right socket, the snug fit ensures that the bolts stay as they should: hexagonally shaped. Despite this, many complete socket sets come with both 6 and 12-point sockets. 12-point sockets do have their advantages, such as having a better range of movement, but on a socket wrench you want a solid grip on the bolt.
12-point box-end wrenches are better than 6-point wrenches, but for sockets, the 6-point variety is superior due to its better grip and preservation of the bolt head.