Overhead transparencies permit the use of overlays to build up a complex diagram, calling attention to each feature as it is added. Transparencies may be colored easily. Water-soluable felt tip pens are good and can be erased. Thus, color highlights can be added to a line-drawn transparency before or during use. Or colored adhesive film can be added to liven a black and white visual aid.
Slides are convenient to show but are less flexible than overhead transparencies.
They are usually more expensive to produce. Handouts should be provided. It is desirable for the instructor to hand out in advance a student outline and copies of visual aids. This will enable students to take notes right on the appropriate page if they wish. Also, students will not have to divert their attention to re-draw the visuals from the screen.