With increasing public awareness of the high cost of living, efficient allocation of limited resources such as money, energy, and time has become essential. Consequently, with the large variation of measures of goods and services available in everyday life, a need has arisen for quick comparison of items of different quantities and even of different measures of value. In daily shopping, for example, where goods are sold in varying quantities of ounces or grams, it is necessary to calculate the price of the goods per ounce or gram in order to determine which brand and which quantity offers the optimum cost per unit of goods. This comparison requires convenient mathematical computation. While some such calculations can be performed mentally, others are too complex to determine without some form of mechanical assistance. Pencil and paper or even a calculator can be of assistance in this situation, but neither is convenient and the quicker of these two, the calculator, is costly and bulky. A third method of calculation is a premade triple scale printed card which allows rapid calculation by placing a straight edge across two scales of the card, intersecting the appropriate numerals corresponding to each product's quantity and cost. At the intersection of the straight edge with the third scale the desired calculation can be read. Performing this process for two products allows a mental comparison of cost per unit of quantity. This method has the disadvantage of requiring a printed card and the secondary device of a straight edge. Carrying these two pieces of equipment during shopping is awkward.
Alignment of the straight edge is also difficult since pressure must be exerted on the straight edge at the first aligned intersection point to prevent it from slipping while the second intersection point is aligned. If the edge slips an inaccurate calculation will be obtained.