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I came across a different symbol for the letter 'S' on Wikipedia:
What is the symbol called, replacing the letter 'S' in "Paradise" and "Lost" (and lower down in many other words, like "Bishopsgate-street")?
Further, why does it only replace some 'S' symbols? It seems that when used for pluralization, the modern 'S' is used. Examples in the image above are "Books" and the first and final 'S' in "St. Dunstons", but not the 'S' in the middle of that word.
What are the rules for when to use one vs the other?
The symbol looks rather like a lowercase 'f'...