Computer software, or simply software is any set of machine-readable instructions that directs a computer's processor to perform specific operations. Computer software contrasts with computer hardware, which is the physical component of computers. Computer hardware and software require each other and neither can be realistically used without the other. Using a musical analogy, hardware is like a musical instrument and software is like the notes played on that instrument. Computer software includes computer programs, libraries and their associated documentation. The word software is also sometimes used in a more narrow sense, meaning application software only. Software is stored in computer memory and is intangible, i.e. it cannot be touched. At the lowest level, executable code consists of machine language instructions specific to an individual processor – typically a central processing unit. A machine language consists of groups of binary values signifying processor instructions that change the state of the computer from its preceding state.