I was fascinated to find a discussion of the difference between a ditto machine and a mimeograph. For what it’s worth, my memory of the difference between the two is the same as Michael Sheehan indicated in his post. The Ditto machine used masters that had a carbon based (I believe) substance on them, and this would be transferred by the machine to the individual copies.
The mimeograph machine used a stencil in which the text or graphics were cut so that ink could seep through, thus getting to the individual copies. You could even do color pages by running more than one stencil using a different color of ink, though the process was difficult.
An interesting variation on ditto, which was done quite cheaply, was a gel filled pan. We would buy teaching materials on masters which could be set on the gel which would take an image of the contents of the page. Then each sheet that was placed on the gel would get a copy of that image. After a few copies, the image would fade. If you waited long enough, you could do this again with the same gel, though eventually it would become too filled with leftover ink to be useful.
The reason I know this is that I worked as a volunteer in my very tiny private school starting when I was about eleven years old and on into my teens. I had learned to type when I was 8 years old, a childhood decision I have celebrated through the rest of my life. My handwriting is nearly illegible, but I can type consistently between 80 and 90 words per minute and in a hurry can attain well over 100 wpm.
The little school used a great deal of material prepared on the spot for their lessons, and there was no money for a regular secretary, so after school each day I would type masters, first the ditto masters, and later mimeograph masters. The ditto masters were a bit easier to handle, but they were very hard to correct. They consisted of a sheet of thick paper attached to a sort of reverse carbon paper. When you typed on the front of the paper, the carbon stuck to the back of the sheet. If you made any errors, you had to use a razor blade to scrape the carbon from the back of the sheet, and then type again. It made me very anxious not to make mistakes!
The mimeograph sheets were harder to handle in the first place. It was quite easy to get stray marks by scratching or folding them. Our machine was a Gestetner, which I believe was regarded as a Cadillac of machines at the time. The stencils were green and waxy. Corrections were made with a fluid which would block the marks of the previous error. Once the fluid dried, you could type something again.
For those used to modern typesetting programs, I must mention that in some cases I was asked to right justify text on these mimeograph sheets which involved counting out characters and adding spaces manually as you typed. It wasn’t fun, and was hard to make look at all decent.
We eventually also did some sheet music this way, using special pens and symbol plates that were placed on both sides of the stencil. Perhaps it would be clearer to say that you would place a rough plastic sheet behind the stencil and then used a guide of the shape (notes and various musical symbols) you wished to draw. After the notes were placed on the stencil I would have to go through and type the words between the musical lines. In that case errors were even more devoutly to be avoided, as too many corrections would make the stencil unusable, at which point the person who did the musical notes would have to redo their part.
In any case, reading this little post really brought back some memories.