ms-help://MS.VSCC.2003/MS.MSDNQTR.2003APR.1033/cpref/html/frlrfsystemdoubleclasstostringtopic3.htm
Double.ToString Method (String)
Converts the numeric value of this instance to its equivalent string representation using the specified format and culture-specific format information.

Remarks
The return value can be PositiveInfinitySymbol, NegativeInfinitySymbol, NaNSymbol, or a string of the form:

[sign]integral-digits[.[fractional-digits]][e[sign]exponential-digits]
Optional items are framed in square brackets ([ and ]). Items containing the term "digits" consist of a series of numeric characters ranging from 0 to 9.

sign
A negative sign or positive sign symbol.
integral-digits
A series of digits specifying the integral part of the number. Integral-digits can be absent if there are fractional-digits.
'.'
A culture-specific decimal point symbol.
fractional-digits
A series of digits specifying the fractional part of the number.
'e'
A lowercase character 'e', indicating exponential (scientific) notation.
exponential-digits
A series of digits specifying an exponent.
Some examples of the return value are "100", "-123,456,789", "123.45+e6", "500", "3.1416", "600", "-0.123", and "-Infinity".