A collection of related things in a specific order. In mathematics, numbers are represented as sequences of digits e.g. bits, decimal digits, hexadecimal digits, etc. There are also sequences of numbers where each number is related to previous numbers, e.g. the Fibonacci sequence. In computing the sequence of instructions that a computer follows when executing a program is called control flow; a sequence of characters is also known as a "(character) string" (e.g. an escape sequence); a sequence of images forms a video; a sound recording is an example of a sequence of samples of an analogue signal. In probability theory, a sequence of events can be described by a Markov chain.