Until the First World War the only effective aids to calculation were the slide-rule, for 1% accuracy, or books of tables of logarithms, roots and trigonometric functions. These were 4 or 5-figure for normal use and 6 or 7- figure for those who needed the extra accuracy. Many ingenious methods were devised for increasing the the length of the slide-rule scale to improve its accuracy: some scales were made spiral and others were wound round a cylinder or printed in multiple parallel lines. But higher accuracy always meant slower calculation.