= Solution
The <echellogram> contains nearly parallel traces, one for each <diffraction order>. Choose the main <echelle grating> dispersion to increase <wavelength> toward the right and the <cross-disperser> to increase wavelength upward. At a fixed horizontal coordinate, order $m+1$ has a shorter wavelength than order $m$, so it lies below order $m$ in this convention.
\Image[/past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2019/iii/paper-338-echellogram.png]
{title=Adjacent orders in a cross-dispersed echellogram}
{description=Order m+1 is bluer than order m at the same main-dispersion coordinate. Within either order, wavelength increases along the trace to the right; the chosen cross-dispersion orientation puts longer wavelengths higher on the detector.}
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