How many spaces after a period?

Via Stuart Celarier who calligraphy and paleography in college and spent quite a bit of time with typography and type design.

According to Robert Bringhurst’s The Elements of Typographic Style, version 2.4 (Hartley & Marks, Publishers, 1992, 1996, 2001), page 28:

2.1.4 Use a single word space between sentences.

In the nineteenth century, which was a dark and inflationary age in typography and type design, many compositors were encouraged to stuff extra space between sentences. Generations of twentieth-century typists were then taught to do the same, by hitting the spacebar twice after each period. Your typing as well as your typesetting will benefit from unlearning this quaint Victorian habit. As a general rule, no more than a single space is required after a period, a colon or any other mark of punctuation. Larger spaces (e.g., en spaces) are themselves punctuation.

So now you know.