Originals should be kept in a safe place in case of loss or damage. Use a horizontal 35mm slide format, vertical slides should be avoided. Due to low ceilings some meeting rooms will be set with rectangular screens instead of square ones. During projection of vertical slides, heads of those sitting in front often block the view of those in the rear. Time is wasted and the train of thought broken if the projector has to be adjusted for vertical slides and then readjusted for horizontal slides.
Horizontal slides make maximum use of the upper part of the screen.
Use duplicate slides if you are going to refer to the same slide more than once. Legibility will be increased if there are contrasts in brightness and tone between illustrations and backgrounds. Color combinations that increase visibility include white on medium blue ,and black on yellow. Never use black on a dark background. Avoid using red and green next to one another.