Not long after I received a version of Eric Gill's Perpetua, I chanced to see an interesting advertisement done in letterpress for a book. Looking closely at the really magnificent presswork, I noticed that the typeface was Perpetua, but the lowercase y had a straight back. Hmm. I went to my reference sources (among them Anatomy of a Typeface by Lawson) and discovered that, indeed, Gill had designed a straight-back y, which he intended to be used inside a word, while the standard y with the curved tail was to be used at the end of a word.

Stephen Moye. Fontograper. Type by Design. p. 167

In Arabic script, letters have different shapes depending on where they occur in a word: in isolation, final, medial or initial position. This is the first time I encounter this concept applied to English letters.