Scale 1 Ruler
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The standard aeronautical scale rule used world-wide, cut from thick Perspex with bevelled edges. The outer edges are marked with nautical miles in 1:500,000 & 1:250,000 scales, on the centre line statute miles and kilometres are marked in 1:500,000 scale. The scale is 10nm wide (in 1:500,000 scale) to aid safety altitude calculations.