All the tips in this thread are great, and a couple are new to me. Here's another one to keep rulers from moving. I use those large clamps found in tool stores...some are spring loaded and some you tighten by squeezing the handles until they lock into position. Those have a release lever on the inside. I avoid the spring ones because of arthritis...too stiff and hard to use for me. The others are wonderful for holding down rulers and even square rulers. One clamp on a lower corner is enough and you can use your hand to hold down the upper corner (on the same side of the ruler). The clamp works great with the 4 by 36 inch ruler. One clamp on the bottom and the other clamp at the top....work well for me.