Originally devised by Iverson as a mathematical notation in the mid-1960s, and only later implemented as a programming language, APL had a meteoric rise in popularity that lasted until the 1980s. The main feature of APL is that it provides a rich set of powerful operators for handling multidimensional arrays, together with the capability for users to define their own operators. The built-in operators are mainly represented by single characters using a special character set. Thus APL programs are very concise and often impenetrable.