In the old days of typewriters using only monospaced fonts in which a period occupied as much horizontal space as any other letter, it was standard to double-space after each one to clearly separate out each sentence from the following one. However, when justified variable-width type is set for printing It has always been standard to use only one space between sentences. Modern computers produce type that is more like print, and most modern styles call for only one space after a period. This is especially important if you are preparing a text for publication which will be laid out from your electronic copy. If you find it difficult to adopt the one-space pattern, when you are finished writing you can do a global search-and-replace to find all double spaces and replace them with single spaces.