Are there supposed to be two spaces between sentences, or one?

[spaceniuzai]

Hi, I have a simple question. Should there be two or one spaces between two sentences? I see both usages happen pretty often. Is there any preference? Thanks.

[Anglika]

The rule is one space after punctuation.

[spaceniuzai]

Thanks for the answer. However, I do see a lot of documents, some of which are very formal ones, use two spaces between sentences.

[Anglika]

Doesn't alter the basic rule.

[Ouisch]

Before computers and word processing programs were the norm and documents were composed on typewriters, the rule was two spaces in between sentences. But since word processing programs use proportionate spacing, one space is now the standard.

[Anglika]

When learning typesetting, using a Monotype machine, one space was the rule. The space width might vary according to the length of the line and the way in which the characters fitted into the width. I was not taught to put in double spacing when learning to type either.

It is odd what becomes a norm and then changes again. With all the insistence on reducing keystrokes, then to require an unnecessary one ... !