Have you been frustrated when trying to make an inside measurement because you just can't get an accurate enough reading? Here's a simple technique that's been used by craftsmen and cabinetmakers for centuries.
Take two strips of wood whose combined length is longer than the distance you want to measure. Set them inside the area you want to measure and then slide them apart until either end comes in contact with the inside edges you want to measure.
Make an index mark.
Remove the sticks, realign the index mark, secure them with a spring clamp and then take your measurement. There's no way to be more accurate.
If you're interested in learning more about measuring sticks, you might enjoy this article from The Wood Whisperer.