At long last I have grasped how dendrochronology actually works. It took an entire classroom of patient colleagues, and an especially lively discussion, to achieve this over lunch today. Dendrochronology is the method of dating the demise of living trees, and pieces of wood cut from them, by counting, measuring, and analyzing the patterns formed by the rings that accumulate naturally through successive annual growth cycles in late spring, when wood grows faster and is less dense than in the fall and winter, and may be clearly seen in cross section through the trunk of a tree, from its bark to its core. Evidence of such tree rings may also be observed and recorded along the edge of planks of wood sawn from individual trees, and although these necessarily tell only a relatively small part of the whole growth history of that larger tree, the unique sequence of seasonal variations that are preserved in such a piece of wood does nevertheless form a reasonably reliable fragment of a botanical fingerprint, as it were, that ought to correspond with contemporaneous growth patterns in comparable pieces of wood from the same species of tree that once grew in the same region—an important caveat, because therefore such trees shared patterns of rainfall and other climatic conditions that governed their growth each calendar year. Previously I could not understand how such ring patterns could provide anything other than relative data, with luck corresponding to otherwise independently verifiable dates that could, in turn, be pinned fairly reliably to particular pieces of wood. But let us suppose, for example, that one is interested in knowing the exact age of a piece of oak that forms part of a fine English panel painting of approximately the late sixteenth century. This is how it is done: We know that supplies of the finest quality close-grained oak were in the late sixteenth century imported to England from the Baltic, and share certain basic characteristics. If you cut down a tree in that region today, right now, and analyze the ring patterns that radiate in reverse chronological order from the middle of the trunk you may trace how the rings correspond with each growth season extending from last year all the way back to the birth of the tree itself, perhaps 300 years ago. That sequence will necessarily overlap with the ring patterns in a piece of wood from a similar tree that was cut down 155 years ago, and was, let us say, at that date 420 years old. For 145 years both were growing at the same time and with the same annual spurts of growth, or enduring the ill effects of the same especially appalling winters, and this therefore yields reliable data that apply to annual seasons through an accumulated total of 575 years. How do we know the older tree was cut down 155 years ago? Because its last growth ring exactly corresponds with the 155th of the younger tree. By this method it is possible to assemble relatively accurate overlapping data, extending back for centuries, with which to compare the sequence of annual tree-ring growth patterns discernible in much smaller pieces of wood. This in turn allows the dendrochronologist to estimate within several years the exact date when each original tree was cut down. There are a number of variables, such as conventions of the Renaissance lumber trade that governed which portions of the tree trunk afforded the straightest and best quality planks, and the length of time such planks were left to season before sale, which usually means that the age of the plank is maybe slightly less than the date when the tree was felled, but greater by a certain number of years, perhaps up to seven, than the age of the panel painting the plank was used to build. Occasionally, other evidence contained within the painting itself may be used to narrow down the available date range afforded by the dendrochronologist, and other documentation about extremes of climate may also help to confirm data that is occasionally otherwise compromised by the action of wood-eating insects, etc. However, we do know that by this method it has been possible to identify certain English panel paintings that were definitely made from planks hewn from the very same tree. This does not necessarily shed any more light upon our painting than that the craftsmen who supplied the artist with his panel sourced their timber from a single batch, which may or may not have kept them supplied with the raw material to make their panels over an additional period of months or even years. Still, it is amazing, because by this method it may be argued that our late sixteenth-century English painting adheres to a panel made from Baltic oak that was felled in 1585, which is what we like to call a reliable terminus post quem. One must be wary, of course, because cunning forgers have at times been easily capable of using an old panel to fabricate a new picture, and pass it off as something else entirely, but that is a very different story.