A check is a term used to describe a crack that runs through a board, usually running lengthwise. It is usually caused when lumber dries out too quickly. Adding a UV protective stain can help protect decking and rails. Checking often affects pressure treated framing materials as they are usually sold very green and are put under a lot of stress as they dry out in the sun. Thicker members like 6x6 posts tend to dry out unevenly and are especially susceptible to extreme checking.