[rg]

And the overhead projector was used in a lighted conference room, so that people could see the presenter and one another and consult papers, and black-on-white was easier to read.

The style of light-on-dark came from 35mm slides in 35mm projectors, almost always used in darkened rooms and easier to read in the dark.

When PowerPoint expanded from acetate overheads to also make 35mm slides, templates in both styles (dark-on-light, light-on-dark) were included. Both styles persist in today's electronic presentations, for use in appropriate surroundings, although it is easy for presenters to make a poor selection.

It was clearer when the rule was that a photocopied transparency measuring 8.5 x 11 inches should be dark-on-light, and a transparency measuring 24mmx36mm in a 2-inch square mount should be light-on-dark.