Silent letter

Silent letters are letters written in words that, when spoken, do not correspond to the pronunciation of a given letter and also do not allow anyone to hear their sound when they appear in the word. 
Many of them come from Old English. These include the "k" in "know" and "knee", the second "b" in "bomb" and "numb," the "s" in "island," and the "e" in "lame" and "lime". There is also a silent g in "sign", which is from Middle English. Old English uses the sound of the silent "k" and "gh" from "knight", which was pronounced at the time. The word changed its history. 
Other silent letters may be the "r" in words from Old French, like "hors d'ouvert" and the sound of the "t" is sometimes heard in some words like "often," but it may also be silent. There is also a silent "w" in "answer", and the "w" is not pronounced. The word "kneel" starts with a silent "k". 
Silent letters exist in the spelling systems in many languages. They are very common in languages with deep orthographies, spelling systems with little one-to-one relationship between letters and sounds, like English, French, Thai, and Mongolian.