casio fx-451m

the innovative design has an additional 32 buttons inside the cover. the buttons operate by touch and make the unit slightly awkward to operate when held by one hand. a weakness of the design is that, after prolonged use, the internal edge connections can fail and the spin can sometimes split. hp came up with a much more elegant way to implement this with their 28c and later models.

nevertheless, the idea provides space for many useful features otherwise not so well accommodated. one such feature is the extensive collections of physical constants and unit conversions.

altogether a very compact and functional unit.