I am a graphic designer by trade. SInce the advent of personal computers and properly designed fonts, it is not longer necessary - in fact it is incorrect to use two spaces after a period. Modern - as in not typewriter - fonts have the correct spacing built into the "period" character. Only once space is therefore necessary.

Two spaces was used back when people typed with monospace fonts, that did not have spacing correctly designed for typography. Each letter had the same with, even an i and an m - which is just silly. That is why you needed two spaces then, because frankly everything was weirdly spaced, and you needed two spaces to make a difference.

That is no longer true.

One space is all that is correct after a period these days.