Slides/transparencies for the overhead projector (OHP)
Names for overhead projector 'slides'
'Slides' is the word for what is shown in modern computerised projection and what used to be shown in early projection with a slide projectors, but I never knew it used for what was shown on the overhead projector (OHP). The terms were either 'transparencies' or 'acetates'.
What transparencies/acetates were
Transparencies or acetates were transparent sheets, although as can be seen in the photo on the OHP page, there was usually a roll of poorer quality transparent material which was designed for single use at the time of projection. The separate sheets were intended for keeping for another presentation.
The coating on the transparencies
The sheets were coated with a type of acetate which held writing without it separating out into globules. This coating came off slightly when handled and it affected the skin. So there was a white strip at the side for handling. It was difficult, if not impossible, to prepare the transparency without handling it outside the special strip, and I for one used used to feel my fingers slightly numbed and uncomfortable afterwards. After a good wash they felt back to normal.
Later, transparencies were developed without the strip, so were presumably deemed to be safe for the skin. Transparencies were also developed which took photocopying.
The pens for preparing the transparencies
Special pens were needed to write on the transparencies. They were available in several colours and thicknesses and came in two types: permanent and non-permanent. The permanent ones always seemed to come with black outsides, so as to tell the two types apart at a glance.
There were advantages and disadvantages to each type. With non-permanent pens, errors could easily and quickly be erased with a damp cloth, so allowing rethinks, but care had to be taken to keep them away from anything wet. Stories got around of teachers carrying them between buildings in the rain and finding to their obvious consternation that they were blank on arrival.
Permanent pens, while not having this disadvantage did not allow for errors or rethinks.
How to make transparencies
It was of course perfectly possible to make a transparency very quickly indeed by simply writing on it with an appropriate pen. However, for special presentations, particularly company meetings, something more professional was expected. (Remember this was at a time when word processors were in their infancy and printers were unsophisticated with only one font size and type at a time.)
How to vary the fonts
One way to get changes in font types and sizes was to use stencils. This was very labour intensive because of having to line up each stencil character with the other characters, and it required special drawing boards with moveable ruler and clips to hold the transparencies in position. Nevertheless it was common practice.
Another way was to use rub-on letters, also known as dry transfers which went by the trade name of Letraset. This was equally labour intensive because of lining up the lettering.
A range of stencils and dry lettering sheets were the tools of the trade for whoever had the task of making professional-looking transparencies. So were the special drawing boards.