Most typographers consider double spaces a quaint holdover from the dark ages of monospaced typewriters- where every letter and punctuation mark occupied the same amount of space. Because of this limitation, the practice of adding double space became helpful in visually separating sentences. Modern word processors use typefaces that are proportionally sized and spaced and do not require extra spacing after the period. Its allowable to use both methods of spacing, but many designers will say that a single space looks better because it mitigates the appearance of "rivers" in a block of text.