Miller's Law describes the observation that the number of items an average person can hold or process in their working or short-term memory is 7 (+ or - 2), making the average range 5-9 items. This was first observed by George Miller in the 1950s. Working or short-term memory is information that you are 'holding' in your mind or currently working on.
Also called Miller's Magic Number, the average person can hold 7 (+ or - 2) items in their mind accurately. A phone number is a common 7 item chunk of information that can be held in your mind. If telephone numbers were 15 digits long it would be impossible for the average person to be able to recall it or hold it in their working memory.