ICAO Chart Rule

ICAO-spec aeronautical charts are produced by most countries for their own aviation purposes. The charts come in three standard metric scales:

1:1,000,000 Scale (World Aeronautical Charts for flight planning)
1:500,000 Scale (VFR Navigation Charts, often called "Sectionals" in the USA)
1:250,000 Scale (VFR Terminal Areas)

This scale is useful for the aviator that uses two or more of these scales. Before the world agreed on a common standard, aviators used to make up terms to describe the scales, such as "8 miles to the inch", but they are now commonly described using their metric titles.