# Why there is a 'MACH' in this Compressibility Correction Chart?

I couldn't find what 'Mach 1.0' and 'Mach 0.8' are trying to indicate in the middle of the chart.

As I read it, the upper arch pointed to by Mach 1.0 represents the correction necessary at the different pressure altitudes when flying at Mach 1.0, while the lower arch pointed to by Mach 0.8 represent the correction necessary at those pressure altitudes when flying at Mach 0.8.