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Floating point numbers are the computer equivalent of scientific
notation. The precise number of significant figures and the range of
possible exponents is machine-specific; Emacs always uses the C data
type double to store the value.
The printed representation for floating point numbers requires either a decimal point (with at least one digit following), an exponent, or both. For example, ‘1500.0’, ‘15e2’, ‘15.0e2’, ‘1.5e3’, and ‘.15e4’ are five ways of writing a floating point number whose value is 1500. They are all equivalent.
See Numbers, for more information.