I am just new to fontlab and just starting to working on my spacing, so this topic also interests me.

My understanding of the measurement line is to help you space your sidebearings correctly. You can move it up and down and whereever it falls on the glyph on both sides of it it will measure out to the sidebars. So if you had a flat side and a fat side like a capital H, you could give it equal distance on either side right in the glyph editing window without going into the metrics (although i find it easier here)

Maybe there is more to it, i am not sure, but looking forward the responses from others :)

That’s basically correct, Thomas. I actually invented the measuring line and persuaded Yuri to implement it, so I guess people can blame me if they don’t like it. The idea though is to provide a means to obtain and manipulate metrics based on distance from stems and bowls rather than from extremes. So, for instance, if you have a lovely oldstyle typeface with lots of irregularities such as serifs of varying lengths, the measuring line is extremely useful, because you can ensure even spacing of vertical stems relative to each other without having to take into account the difference in serif length when setting sidebearings. Similarly, if you have bowls with slightly different curvature, giving them all the same sidebearing value won’t necessarily produce even spacing. But if you set the measuring line at around half the x-height, or just slightly higher, and then set the measuring line to sidebearing distance to be the same for all bowls of the same height you will generally find the spacing to be more even.

Sidebearings, as a measurement from the left and right extremes of a glyph, are a mechanical setting, which has nothing to do with how one actually spaces letters, which has to do with the relationships of white space between stems and bowls. Being able to measure off the stems and bowls directly is very helpful.

Note that in FL Studio 5, the height of the measuring line is saved with the VFB file, which means that it won’t keep jumping back to 300 units high every time you re-open FontLab. I generally set the measuring line relative to the x-height, for a Latin typeface, and lock it in that position throughout development, using it extensively for spacing. The only time I’ll shift it is if I want to use it specifically for spacing uppercase letters.

A couple of additonal comments:

I make most use of the measuring line within the metrics window, where you can turn it on and set values relative to stems and bowls just as you would relative to extremes.

Note that in the Glyph Properties panel, two sets of horizontal metrics are shown for the selected or open glyph: actual sidebearings from extremes and also sidebearings at the measuring line height.

I shall have to read attentively this invaluable information to understand quite the subtleties of this tool. Thanks

And don’t forget italics. For me, the measurement line is essential for getting italic spacing in shape.

This measurement line is very useful, but can we mask it or hide it? Only for some time.