According to this link, we are missing (in Modern English) at least three letters that used to be in common use in English. These are thorn, edh, and yogh. Are there others that were clearly in the ...
Is the word formulæ, written with an æ at the end, valid in English? I stumbled upon this apparently plural form of formula in the Wiktionary. I had no idea the letter æ could occur in English. Does ...