Two spaces after a period: Why you should never, ever do it. – Slate Magazine.
This article deals with a mistake I dealt with in teaching. Students are taught in high school, if not earlier, to put two spaces after a period, a problem when using variable width fonts. Teaching technical writing, I stressed getting rid of the extra spaces, showing students why it is a problem. Their scores on reports were not lowered, however, if they did put two spaces after periods.
In teaching page layout and design, I did penalize scores for the extra spaces and asked for projects to be revised. As the article says, on typewriters, with fixed width fonts, the extra space helps. But fixed width fonts make for amateurish appearance.
Some submissions to The Ghazal Page have two spaces after periods. I ignore those, since browsers parsing HTML ignore the extra space.
As I told my students, contemporary word-processing software allows you to create (almost) professional layouts. Why look like an amateur?