On The Atlantic, Lindsay Lynch writes about typesetting letterpress and the en space in “How I Came to Love the En Space”:
To understand letterpress printing, imagine that every letter you see on your screen is an object, a tiny piece of metal. Not only is every letter an object, but every space between every letter is also an object. Every space between words, every space between lines—every bit of white space is an object. When typesetting, a printer has to think about negative space as something tangible.
This is where the en space comes in. An en space is a rectangular piece of metal or wood whose primary purpose is to be smaller than the metal or wood type being printed. The en space isn’t type-high—it doesn’t sit proud like an ordinary character—so it doesn’t catch ink when it’s run through the press. It just holds printable type together in a tight grid, creating spaces between words. It is never seen, but without it, everything printed would be nonsense.