In order to press the blocks onto the paper, they would have to be turned upside down. What kept them from falling out? I think I’m making sense, I’m trying to ask this question without knowing any of the terminology.
Quick answer: The type blocks, engravings, and whatever were “locked into the chase” using a quoin. Basically, imagine a frame that when tightened up around the blocks, applies sideways pressure to them to hold them in.
However, the chase was not turned upside down; rather the paper was pressed onto the type using rollers from above.
This is a quick, down-and-dirty answer. Try searching on “chase,” “quoin,” and “letterpress” to get started. I’m sure someone with direct experience will be along to explain in more detail.
Well, it will depend on when you are asking about. More recently (last 100 years or so) it was a spring loaded spacer thingy. Before that it was a screw-type spacer thingy that put tension on the blocks to keep them in place and before that it was blocks of wood and shims hammered in.
In each case the tension is created from the sides to keep them from falling out (forward) when the paper was applied. Something like carrying three or four books side to side by only pressing in from the two outside books.
I should add that in many if not most cases of letter press the blocks are facing up and the paper is applied on to it thus also lowering the chance of blocks falling out.
At the print shop i used to work at, they had this terrific tape from 3M. It wasn’t really tape, more just tape adhesive on wax paper. They would use it to glue the plates, (large, thin sheets of metal photoengraved) on to the rollers. I still have a partial roll of the stuff. Great contact adhesive…
These are all great answers, thanks for making them so speedy. I did try the search and found a lot of stuff.