On a standard 1:500,000 aviation sectional chart, the map is broken down into quadrangle "sections" with a maximum elevation figure printed in blue in the center.
Using a chart-plotter I can see that the vertical (north-south) grid lines printed on the chart are in units of nautical miles. Each quadrangle is 30 nautical miles vertically, and roughly 20.5 nautical miles wide (west-east).
What's confusing me is that the scale printed on the horizontal portion of the quadrangle section doesn't appear to be in nautical miles, statute miles, or even kilometers. What does the horizontal scale represent? Is there a unit associated with the horizontal grid?