Which letter of the modern alphabet eventually replaced the Middle English letter called thorn?

Have you ever wondered why quaint, old-timey shops sometimes have names beginning with ‘Ye Olde...’?

The answer is the Middle English letter ‘thorn’. Thorn or þorn (Þ, þ) is a letter which is found in Old English, Middle English, Gothic, Old Norse, Old Swedish, and modern Icelandic alphabets. Somewhat resembling a cross between the letters p & b (or possibly a pregnant capital letter I) the thorn makes a ‘th’ sound; which means ‘Ye Olde Gift Shop’ (with a thorn rather than a Y at the beginning), pronounced properly in Middle English, would have been ‘The Olde Gift Shop’.

Though the ‘th’ sound the thorn made has endured, the letters for Y and thorn looked very similar in certain Gothic scripts. In some hands, such as that of the scribe of the unique mid-15th-century manuscript of ‘The Boke of Margery Kempe’, it ultimately became indistinguishable from the letter Y. Eventually, the substitution of Y for Þ became the norm, which leads us to today’s ‘ye’, as in 'Ye Olde Curiositie Shoppe'. Another cause of the thorn’s elimination and near-extinction, was the printing press. The letter Y was found in the printer's type fonts that were imported from Germany or Italy, but Þ was not, so Y was used. The thorn fell out of use in most alphabets, and today the letters ‘th’ are used to replicate the sound.

Today, some modern languages, like Icelandic, still retain the thorn in its original usage and form.