Wood is a hygroscopic material which means it absorbs and desorbs water from the surrounding air. This change in moisture content causes the wood to expand and contract in relation to the relative humidity and temperature of the environment the wood resides in. When we build furniture we must allow for this movement because you cannot stop it. Finishing the wood will slow the moisture transfer down some, but no finish will stop it. Most if not all wood deformation can be trace to this expansion or contraction due to moisture. Understanding Wood Movement is one of the gateway skills of wood working.
I got the idea for this post while reading on one of the forums. I realized how much poor information is prevalent on the forums regarding the subject of wood movement. While researching for this post, I realized that many others have addressed the subject quite well. My rehashing things that have already been covered well is just a waste of time, so I have decided to create a list of links related to the subject of wood movement that cover good information not the numerous wives tales you read on the web. If you know of a good link I have missed, leave it in a comment. I view this post as a work in progress and will edit it as needed.