Slides can, in principle, enhance a lecture, but there is one important difference between slides and blackboard that definitely needs to be kept in mind, and that is that slides are much more transient than a blackboard. Once one moves on from one slide to the next, the old slide is completely gone from view (unless one deliberately cycles back to it); and so if the student has not fully digested or at least copied down what was on that slide, he or she will have to somehow try to catch up in real time using the subsequent slides. Often, the net result is that the student will become more and more lost for the remainder of the lecture, or else is spending all of his or her time transcribing the slides instead of listening in real time.

In contrast, given enough blackboard space, the material from a previous blackboard tends to persist for several minutes after the point when one has moved onto another blackboard, which allows for a less frantic deployment of attention and concentration by the student.

If one distributes printed versions of the slides beforehand, then this difficulty is mostly eliminated. Though sometimes it takes a few lectures for the students to adapt to this. Once, in the first class in an undergraduate maths course, I said that I wanted my students to try to understand the lecture rather than simply copy it down, and to that end I distributed printed copies of the slides that I would be lecturing from. (The slides were in bullet point form, and I would expand upon them in speech and on the board.) I then found that for the first few lectures, the students, not knowing exactly what to do with their time now that they did not have to take as much notes, started highlighting all the bullet points on the printed notes. It was only after I threatened to distribute pre-highlighted lecture notes that they finally started listening to the lecture (and annotating the notes as necessary).