Silent letters, also referred to as non-vocalized letters or simply non-vocal letters, are consonants written in words that, when they are spoken, do not correspond to the pronunciations of given letters and also do not allow anyone to hear their sounds when they appear in words.

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 words, such as knowledge, also begin with silent k when the letter n comes after it. There is also the silent r that happens to be in many other words from Old English, such as the one that happens to be heard in former words such as February that can be sounded and sometimes it is also silent unlike how you kneel as well as that silent d in Wednesday that happens to be the first as well as that. There is also the silent n at the end of other words, such as autumn. The word wrong means not right, and begins with silent w. There are words that sometimes begin with silent g, such as gnat.

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. The letter g has two sounds, both hard and soft, but it is sometimes silent at the beginning of the word gnat when it comes before the letter n that comes after it. You can sometimes say the silent g for fun in order to start a word, such as gnome or gnat because they may be completely different. That is why the completely silent g in gnat happens to be quite different from other consonants, and you can only believe that it is near what you might keep remembering after you remember. Also, gnome is very different from that. One word, which is gneiss, is written like nice, but it is only a particular rock that starts with that silent g before that n.

Silent letters exist in the spelling systems in many languages. They are very common in ones with deep orthographies, or spelling systems with little one-to-one relationship between letters and sounds, which include English, French, Thai and Mongolian.