As is well known, fasteners such as machine screws, bolts, and the like are applied and removed by means of such tools as open-ended wrenches, box wrenches, socket wrenches, and Allen wrenches which may be operated by hand or by means of suitable power equipment.  Oftentimes the tools are carried in tool boxes or cribs and the fasteners in various containers, and it is a matter of common experience that it is frequently necessary to fish around in the tool box to find a tool appearing to match the size of the fastener being applied or removed, only to find that the tool is of the wrong size so that this time-consuming and frustrating procedure may have to be repeated a number of times until the tool has been found that matches the size of the fastener.  Even though the tools may have their sizes marked on them, the tools often become jumbled up in the tool box or crib and thus it is troublesome to find the appropriately sized tool.  Complicating this problem is the fact that oftentimes the operator is working in dimly lit areas so that such size markings cannot readily be seen.  Moreover fasteners usually do not have their sizes marked on them and thus the operator has to make a guess as to the proper size of tool needed. 
The resultant cumulative loss of time, especially on large sized or repetitive jobs such as in manufacturing or assembly plants, on construction sites and in repair shops can be extremely costly and constitutes a serious loss of productivity.