This is the 4th version of an old tool that I use all the time: 1) was my Fathers old heavy '50s heavy cast aluminum level that I inherited. I'd put blocks on the board at end, spanned the fretboard with the level, and clamp the neck into a back-bow for adjustment — "helping the truss rod do its work". Recently my pal Dave Flis made a printable version and sent it to me (I think he will also send it to loothprints if he hasn't already). While I am sure to find a good use for Dave's cauls, it isn't what i needed for my job. Rather than modify his we made our own ("we" being Andre Hallenburg, my trusty shop-mate). This version only rests over two string in the center so one can look down the fretboard to see the back-bow happening.