For example, one section of the board might be reserved for a topical outline or an important graph to which you refer throughout the class period. You could plan to fill other sections of the board from the top down with supporting information or drawings that follow a sequential pattern, then erase them as you need more space. Students will usually copy everything you write on the board, so you need to allow time for this, especially if you use pictures or diagrams that you alter as you go along. Instead of erasing and redrawing,use dotted lines or colored chalk to show changes so students will be able to follow your explanation better. The details in complex drawings are often illegible on a chalkboard, so it is better to use another medium, such as an overhead transparency, for a complex image.
Most people have little difficulty using overhead projectors because the technology is very simple.
Some teachers use the projector as a surrogate for the blackboard, writing and drawing on clear acetate sheets with colored pens, but the medium has many advantages over the traditional blackboard. Transparencies can be prepared ahead of time to present more complex and polished images than are possible if drawn on the spot. Some teachers also duplicate the transparencies on handouts to insure that students get complete and accurate images. This step also insures that students don't have to divide their attention between copying a diagram and listening to the teacher's explanation. Transparencies can be drawn by hand with colored pens on clear acetate, but the common practice is to reproduce material on a copy machine that accepts acetate.