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Floating-point numbers are the computer equivalent of
scientific notation; you can think of a floating-point number as
a fraction together with a power of ten. The precise number of
significant figures and the range of possible exponents is
machine-specific; Emacs uses the C data type double
to store the value, and internally this records a power of 2
rather than a power of 10.
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’, ‘15.0e+2’, ‘+1500000e-3’, and ‘.15e4’ are five ways of writing a floating-point number whose value is 1500. They are all equivalent.
See Numbers, for more information.