The tongue and groove joints offer a means of registering the joint edges during assembly. They are often used without any glue, allowing the boards to expand and contract without any negative effects. As long as the contraction of the board is less than the length of the tongue, the joint will not be exposed, and the panel will retain its intended appearance.
The joint is formed by having one piece having a groove, or slot, cut the length of the edge. This groove is most often one third of the wood's thickness and is placed in the center of the edge, producing two walls of wood that are the same thickness. The other piece has the sides of the stock removed, leaving a tongue that is precicely the width of the groove formed on the first piece.
The recommended length of the tongue depends on the width of the stock used to form the panel. For panels formed with stock less than 3 inches wide, the tongue length is not that much of a factor. For these panels, the tongue only needs to be as long as they are thick. This will produce a tongue that appears square when viewed from the end. For panels that are formed with wider stock, it is recommended that you make the tongue's length at least half the stock's thickness.
The groove should ALWAYS be slightly deeper than the tongue is long, by as much as 1/16" for 3-inch wide boards. The reason for this is two-fold. First is to prevent problems during assembly. If the tongue length is cut exactly to the groove depth, then the slightest piece of sawdust or imperfection in the wood will keep the two pieces from mating properly. The second is because of the effects of seasonal expansion and contraction. If one panel expands at a slightly different rate than its neighbour, the tongue from one piece can actually push its neighbour away, and break the joint.
When all the tongue and groove boards in a panel are assembled, there is often a slight difference in height between the panels, or the panels may separate slightly due to seasonal changes, and that can produce an effect that is undesireable for some people. In those cases, you can add a tiny bevel on the edge of every board. This will produce a v-groove effect between each board, and it will camouflage the uneven height, at the expense of having a visible groove.