Barry is right on except that I judge relief with feeler gauges. You can feel with the gauge is contacting the string or fret, you can try rocking it slightly, you can very lightly press the string down to see if it moves or the feeler gets tighter. Frankly a couple of thousands doesn't matter.

(It probably helps that I grew up with car engines that actually needed to have spark plugs and breaker point gaps set as part of tuning them)

But sparkplugs don't flex like guitar strings...

Breaker points were every bit as troublesome as strings to me...


Pierre

I use feeler gauges for measuring relief. Because the strings do flex, I look for movement of the string as I slide the gauge between it and the fret. The thickest gauge I can get in the gap without moving the string is the what I consider as the relief. I do this with the guitar on my workbench, not in a playing position. I use a small spacer to lift the outer strings a hair as I measure the inner strings. I haven't found that negligible amount of added tension to create any problems.