Editing Slide rule (section)

==Basic concepts==
[[File:Slide rule-homemade teaching base 2.jpg|thumb|Simple slide rule made from [[index cards]] marked with powers of 2, calculating 8x4 by aligning the bottom ruler to start where the top ruler is 8, and then reading at the number above where the bottom ruler is 4.]]Each [[Slide rule scale|ruler's scale]] has [[Graduation (scale)|graduations]] labeled with [[precomputed]] outputs of various [[mathematical functions]], acting as a [[lookup table]] that [[Mapping (mathematics)|maps]] from position on the ruler as each function's input. Calculations that can be reduced to simple addition or subtraction using those precomputed functions can be solved by aligning the two rulers and reading the approximate result.

For example, a number to be multiplied on one [[logarithmic-scale]] ruler can be aligned with the start of another such ruler to sum their [[logarithm]]s. Then by applying the [[List of logarithmic identities#Logarithm of a product|law of the logarithm of a product]], the [[Product (mathematics)|product]] of the two numbers can be read. More elaborate slide rules can perform other calculations, such as [[square root]]s, [[exponential function|exponentials]], and [[trigonometric function]]s.

The user may estimate the location of the decimal point in the result by mentally [[Interpolated|interpolating]] between labeled graduations. [[Scientific notation]] is used to track the decimal point for more precise calculations. Addition and subtraction steps in a calculation are generally done mentally or on paper, not on the slide rule.

=== Components ===
[[File:slide rule cursor.jpg|thumb|Cursor on a slide rule]]

Most slide rules consist of three parts:
* Frame or base{{snd}} two strips of the same length held parallel to form a frame.
* Slide{{snd}} a center strip that can move lengthwise relative to the frame.
* Cursor, runner or glass{{snd}} an exterior sliding piece with a hairline for accurately reading and aligning numbers. 

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 (which can usually be pulled out, flipped over and reinserted for convenience), still others on one side only ("simplex" rules). A sliding [[Wikt:cursor|cursor]] with a vertical alignment line is used to find corresponding points on scales that are not adjacent to each other or, in duplex models, are on the other side of the rule. The cursor can also record an intermediate result on any of the scales.

=== Decades ===
[[Slide rule scale|Scales]] may be grouped in [[Decade (log scale)|decades]], where each decade corresponds to a range of numbers that spans a ratio of 10 (i.e. a range from 10<sup>''n''</sup> to 10<sup>''n''+1</sup>). For example, the range 1 to 10 is a single decade, and the range from 10 to 100 is another decade. Thus, single-decade scales (named C and D) range from 1 to 10 across the entire length of the slide rule, while double-decade scales (named A and B) range from 1 to 100 over the length of the slide rule.