Never use more than one word space (space bar) after a word or period. This rule is without exception and agreed upon by all major style guides and typographers: prefer I came. I saw. I conquered. to I came. I saw. I conquered. The habit of double-spacing after a period comes from old typewriter habits and monospaced fonts, yet even during this time you will not find published writing that adds a second space after the period. Extra spaces create unbalance in the text and draw unnecessary attention to their heavier emptiness. Any writer, even ones not accustomed to adding two spaces, should run a word search to remove all double spaces. Like paragraphing, word spacing should remain consistent.