Double Check calculators include the basic arithmetic operations as well as the possibility to calculate percentages, extract square roots or raise a number to a given power. This financial calculator combines commercial functions and conversion functions in one device. It is designed for the visually impaired. It comes in 3 flavours – Double Check (speech output), Double Check XL (extra-large display and buttons) and Double Check XL Voice (extra-large display and buttons and speech output). Spoken functions are with a clear, natural language in 6 volumes. Power is supplied from AAA batteries.