One space between sentences

Some lessons on this website will involve discretionary choices. This one is mandatory.

You must always put exactly one space between sentences.

I understand that many people were taught early in life to double-space their sentences. I was too. But double-spacing is a habit held over from the typewriter age. It has never been part of standard typography. Because typewriter fonts were unusually proportioned, a double space helped set off sentences better. Today, since we don’t use typewriter fonts, double spaces aren’t necessary or desirable.

Let’s see that paragraph again, but with double spaces:

I understand that many people were taught early in life to double-space their sentences.   I was too.   But double-spacing is a habit held over from the typewriter age.   It has never been part of standard typography.   Because typewriter fonts were unusually proportioned, a double space helped set off sentences better.   Today, since we don’t use typewriter fonts, double spaces aren’t necessary or desirable.

Do you see the problem? The extra spaces between sentences disrupt the overall balance of white space in the paragraph.

I understand that many people were taught early in life to double-space their sentences.   I was too.   But double-spacing is a habit held over from the typewriter age.   It has never been part of standard typography.   Because typewriter fonts were unusually proportioned, a double space helped set off sentences better.   Today, since we don’t use typewriter fonts, double spaces aren’t necessary or desirable.

And one more time, in a typewriter font, the one case where double-spacing is tolerable:

I understand that many people were taught early in life to double-space their sentences.  I was too.  But double-spacing is a habit held over from the typewriter age.  It has never been part of standard typography.  Because typewriter fonts were unusually proportioned, a double space helped set off sentences better.  Today, since we don’t use typewriter fonts, double spaces aren’t necessary or desirable.

This is a rule that many nonprofessional writers resist. I’m not clear why that is. But you don’t have to take my word for it—pick up any newspaper, book or magazine and tell me how many spaces there are between sentences.

Correct—one.