A data selection and decoding system useful for selective analysis and speedy assimilation of a large volume of data, in which the data, is set out as an array of code characters. A decoder mask is used to read data from the array, the mask having a viewing slot for viewing a chosen line and bearing, alongside the slot, decoding keys. The mask is made up of sheet portions, each with a viewing slot and possibly bearing decoding keys on each side. By folding the mask so as to reveal the sheet portion or the side of a sheet portion appropriate to a chosen row or column, characters, visible through the slot, can be quickly decoded.