Memory span is the most commonly used measure of short-term memory and refers to the number of words a person can recall in order immediately after hearing them. The short-term memory store, as outlined by Atkinson and Shiffrin, holds information in a verbal form and relies on the process of rehearsal to maintain information in the store. Research generated by the modal model paid lip-service to the idea that short-term memory played a role as a working memory system in more complex tasks, such as reading and mental arithmetic. Baddeley and Hitch explicitly set Hitch explicitly and, in the process, their classic work highlighted certain shortcomings of the modal model. Articulatory suppression involves the subject having to intone some irrelevant words such as 'the, the, the' while holding other words in short-term memory. The central executive in the Baddeley and Hitch model is considered to function as a control system.