Editing Nomogram (section)

==Description==
[[Image:Components of a Nomogram.png|right|300px|thumb|Components of a parallel-scale nomogram]]
A nomogram for a three-variable equation typically has three scales, although there exist nomograms in which two or even all three scales are common. Here two scales represent known values and the third is the scale where the result is read off.  The simplest such equation is ''u''<sub>1</sub> + ''u''<sub>2</sub> + ''u''<sub>3</sub> = 0 for the three variables ''u''<sub>1</sub>, ''u''<sub>2</sub> and ''u''<sub>3</sub>. An example of this type of nomogram is shown on the right, annotated with terms used to describe the parts of a nomogram.

More complicated equations can sometimes be expressed as the sum of functions of the three variables. For example, the nomogram at the top of this article could be constructed as a parallel-scale nomogram because it can be expressed as such a sum after taking logarithms of both sides of the equation.

The scale for the unknown variable can lie between the other two scales or outside of them. The known values of the calculation are marked on the scales for those variables, and a line is drawn between these marks. The result is read off the unknown scale at the point where the line intersects that scale. The scales include 'tick marks' to indicate exact number locations, and they may also include labeled reference values. These scales may be [[linear]], [[logarithmic scale|logarithmic]], or have some more complex relationship.

The sample isopleth shown in red on the nomogram at the top of this article calculates the value of ''T'' when ''S''&nbsp;=&nbsp;7.30 and ''R''&nbsp;=&nbsp;1.17. The isopleth crosses the scale for ''T'' at just under 4.65; a larger figure printed in high resolution on paper would yield ''T''&nbsp;=&nbsp;4.64 to three-digit precision. Note that any variable can be calculated from values of the other two, a feature of nomograms that is particularly useful for equations in which a variable cannot be algebraically isolated from the other variables.

Straight scales are useful for relatively simple calculations, but for more complex calculations the use of simple or elaborate curved scales may be required. Nomograms for more than three variables can be constructed by incorporating a grid of scales for two of the variables, or by concatenating individual nomograms of fewer numbers of variables into a compound nomogram.
[[File:Pythagorean_means_nomograms.svg|thumb|300px|Nomograms to graphically calculate {{nowrap|arithmetic (1),}} {{nowrap|geometric (2)}} and {{nowrap|harmonic (3)}} means, ''z'' of {{nowrap|1=''x''=40}} and {{nowrap|1=''y''=10}} (red), and {{nowrap|1=''x''=45}} and {{nowrap|1=''y''=5}} (blue) &ndash; the arithmetic and harmonic means use linear scales while the geometric mean uses logarithmic scales]]