Measurement line is an optional horizontal line you can use to force FontLab VI to calculate sidebearings at a particular position, instead of from the farthest extremes of the glyph.
Use View > Measurement Line to show or hide the measurement line.
By default, the sidebearing values are measured from the rightmost and leftmost points of the glyph. But sometimes, you may prefer to measure sidebearing values from some other point on the glyph outline (perhaps you want to avoid including serifs, or take the midpoint of an outer diagonal, or....). In FontLab VI, the measurement line defines the “height” at which FontLab VI measures the sidebearings, instead of using the right and left extrema.
You can drag the measurement line to any position, and the glyph metrics (left and right sidebearings) in the Glyph Info panel, in the [Metrics Table](Using the Metrics Table] and in the Property bar will be updated accordingly.