What units are used for the horizontal grid lines on a sectional?
One tick mark along a horizontal grid line on a sectional chart denotes one minute of longitude. At the equator (but nowhere else), this is also equal to one nautical mile.  The further from the equator, the less the distance that is spanned by one minute of longitude.

One tick mark along a vertical grid line on a sectional chart denotes one minute of latitude, which is also equal to one nautical mile, anywhere on the earth's surface.

(The above comments assume a perfectly spherical earth.  Therefore they are not completely accurate, but they are close enough for nearly all aviation purposes.)

It is important to understand that at any given point on a sectional chart-- in fact, at ALL points on a sectional chart-- horizontal and vertical distances are portrayed at the same constant scale.  If you want to measure a given horizontal (or diagonal) distance in nautical miles, you can mark it off on a piece of paper and then hold it against the tick marks on a vertical grid line to get the distance in nautical miles.  Or you can just use a chart plotter.

For larger-area charts, it is necessary to vary the horizontal and vertical scales, to accomodate the curvature of the earth.