The S, T, and ST scales are used for trigonometric functions and multiples of trig functions, for angles in degrees. Using a slide rule for calculation Multiplication Ī logarithm transforms the operations of multiplication and division to addition and subtraction according to the rules log ( x y ) = log ( x ) + log ( y ) and square it using the A and B scales as described above. The cursor can also record an intermediate result on any of the scales. A sliding cursor with a vertical alignment line is used to find corresponding points on scales that are not next to each other or, in duplex models, are on the other side of the rule. Some slide rules ("duplex" models) have scales on both sides of the rule and slide strip, others on one side of the outer strips and both sides of the slide strip, still others on one side only ("simplex" rules).
![slide ruler slide ruler](https://p7.hiclipart.com/preview/433/377/301/slide-rule-ruler-logarithm-calculation-mathematics-rule.jpg)
The outer two strips are fixed so that their relative positions do not change. The strips are aligned in parallel and interlocked so that the central strip can be moved lengthwise relative to the other two. Most slide rules have three linear strips of the same length. Addition and subtraction steps in a calculation are generally done mentally or on paper, not on the slide rule. Scientific notation is used to track the decimal point in more formal calculations. The user determines the location of the decimal point in the result, based on mental estimation. Numbers aligned with the marks give the approximate value of the product, quotient, or other calculated result. The relative position of other marks can then be observed. Mathematical calculations are done by aligning a mark on the sliding central strip with one on one of the fixed strips. The use of slide rules continued to grow through the 1950s and 1960s even as digital computing devices were gradually introduced but around 1974 the pocket calculator made the slide rule largely obsolete and most suppliers left the business. Before electronic calculators were developed, slide rules were the tool used most often in science and engineering. The slide rule is based on the work on logarithms by John Napier and was invented by William Oughtred. Those slide rules have special scales for those applications, as well as normal scales. Some slide rules have been made for special use, as for aviation or finance. These scales are used for mathematical computations. They have a standardised set of markings or scales. There are many different styles of slide rules. The slide rule is used mainly for multiplication and division, and also for "scientific" functions such as roots, logarithms and trigonometry, but usually not for addition or subtraction. The slide rule, or slipstick, is a mechanical analog computer.
![slide ruler slide ruler](https://i0.wp.com/breakingthecode.ca/wp-content/uploads/2015/02/slide-rule.jpg)
A typical ten-inch student slide rule (Pickett N902-T simplex trig)