Trees grow tall from the top (and from the tips of their branches) and out from the bark. That is, trees attempt to apply a sheath-like cone of new wood each year over what existed from the prior year, as illustrated above. The science of dendrochronology pertains only to the radial ring growth as visible from the transverse--or cross-sectional--view, the view typically seen on a cut stump in the wood. To dendrochronologically sample a tree, pencil-thin increment cores are collected from near the base of the tree, and these cores present a single radius of growth for analysis. Cores are collected using hollow increment borers that are drilled into the tree from the bark to the pith, as illustrated above. This type of sampling causes reasonably little harm to the health of the tree. After proper storage (mounting into protective holders) and preparation (sanding or planing the surface) of the sample, the transverse view of radial ring growth is visible.
The crossdating applet randomly generates a virtual increment core of rings for skeleton plotting and crossdating. From the example above of 31 rings, note that the virtual tree growth extends temporally from left to right across the computer monitor. The first ring is marked with an open circle and is considered ring #0, and then every 10th ring is dotted and labeled. Note that some rings may be quite narrow and therefore difficult to see without magnification. Accordingly, the applet allows you to magnify or minify the core image, so if you think you can't see 10 rings between dots, take a closer look by moving the rings that you want see better in the center of view (as marked by a dark blue vertical line over the core mount) and then changing the magnification setting.
The default applet setting for number of rings is to start with 61 rings, which is too many to all fit onto most monitors. You will be able to move the entire virtual core sample left or right with you mouse to see all of the rings. You may also change the number of rings (from as few as 11 to as many as 401) and try different crossdating problems. Beware that the fewer rings you have, the more difficult it is to confidently crossdate because there will be less growth variation to pattern match. This situation is faced commonly in dendroarcheology, for example. In any case, the master chronology with which you can crossdate your skeleton plot of the core will be approximately six times longer that the core series, and the true crossdated match of the sample falls somewhere within the master chronology.