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 Decimal units are unitless — that is, they’re not based on any particular real-world unit, although this is the type used for metric drawings. With decimal units, each unit in the drawing could represent an inch, a millimeter, a parsec, a furlong, a fathom, a cubit (should you be into building arks in case that super rainy day comes), or any other unit of measure you deem suitable, from Danish alens to the Prussian zoll. An example would be 15.5.

 Engineering units are in feet and inches and use decimals to represent partial inches: for example, 12′3.5″.

 Fractional units, like decimal units, are unitless and show values as fractions rather than decimal numbers: for example, 15 1/2.

 Scientific units, which are unitless and display values as exponents, are used for drawing really tiny or really large things. If you design molecules or galaxies, this is the unit type for you. Examples are 15.5E+06 (which is 15,500,000) and 15.5E–06 (which is 0.0000155).

ssss1 The Drawing Units dialog box.