Solid wood is sensitive to changes in moisture content and it will grow and shrink accordingly, more in terms of width than length. If you move your table from the rain forest (damp air) to the desert (dry air), it will shrink. If you move it from the desert to the rain forest, it will grow in size.
I have made many tables in my lifetime and I learned early-on that it is extremely inadvisable to fasten a solid wood table top rigidly to the table's apron. The wood will move over time and if the movement is significant, it will tear the apron and the table apart. That is why I make all of my tables with a dado groove near the top, inside of the apron all around. I then manufacture wooden clips that will attach the top to the apron. The clips are screwed to the bottom of the top. A "finger" is made by cutting away part of the clip at one end. This "finger" is inserted part way into the dado in the apron before being screwed to the table top.
Because the screw holes in the clips vary slightly in position from one clip to the next, I make a habit of marking the clips in numerical sequence with a felt tip pen. I mark the same number under the table top where it will be covered by the clip. This way, the top can be removed for refinishing and be replaced with all the holes holes lining up with the clips.
Movement of solid wood table tops can also be seen in the form of warping or cupping. 1 1/2 "x 2" cross bars can be mounted across the grain, under the top. They should be mounted with # 14 round head brass screws (with brass washers) screwed into the bottom of the table top. The holes through these cross bars must be slots to allow for wood movement .. Never glue these cross bars to the table top, as that will restrict the top's natural movement. Three cross bars should be sufficient for the average dining table, two for a coffee table. The cross bars can easily be removed and replaced for table top refinishing.
Source by Bob Gillespie