If you look at the rings on a tree stump or log, you will notice that some are thicker than others. Wet seasons tend to form thicker rings, while droughts form very thin rings. If a 300 year old tree is cut down, we can look at the pattern in the rings, picking out good and bad years. Then, if we find a log in a historic build that was built 400 years ago, we may be able to match up the patterns, which will take our sequence back further. From there, we look for older structures to extend the sequence further and further.