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’, ‘15.0e2’, ‘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.