Figure 1 - Open end wrench
Figure 2 - Socket
Knowing what size wrench or socket to use on a bolt can save hours of time and frustration. The following chart is applicable for hex, square and lag bolts.
The chart is applicable 99% of the time for open end, box end, 6 and 12 point sockets.
In some cases a manufacturer may adjust the head size of a bolt in order to have the head fit into a certain location.