Coloured overlays in A4 size (11.75" x 8.5") can be preferable to the smaller, more convenient reading rulers in a number of cases, either where factors of age (elderly or young), dyspraxia, or other disability cause co-ordination problems which make it difficult to move a reading ruler down the page to track text; or when both hands are engaged while text or other representations are being followed. Common examples of this use of tinted overlays are sheet music and knitting or other machining patterns.
