If you look at the end of a board and see that the rings run across the width, that board will warp. Perhaps not much, but in humid weather the rings will curl tighter, and in dry weather the rings will flatten out. Since the warping is predictable, you can arrange the boards to cancel out the cumulative effect. Alternating the heart side down and heart side up will ensure that one cups up while the next cups down in a gentle undulation.
Clamping two boards together and jointing them at the same time is another error-canceling practice. Set the boards together, face to face, with the edges you're joining facing upward. If you now shoot these edges simultaneously with the jointer, any tilt of the plane will place identical slopes on both pieces. Folding this joint together will make the edge slopes cancel each other out and the broad faces lie flat. This practice has a price. Planing two boards at once cancels out angular error but doubles crown error.
The rubbed glue joint permits neither error. If you've ever felt how tightly two wet panes of glass will stick together (they have to be slid apart), then you understand the power of the rubbed joint. Like the glass, the boards in a rubbed joint must be jointed dead flat. You can test the accuracy of the jointing by clamping one board edge up on the face of the bench and setting the other atop it. You can see right off if the faces are not in the same plane or if there are gaps along the way. Pivot the top board around its middle to see that it drags evenly. If the top board pivots outward with too little friction, there is crown in the middle. Too much friction or catching on the return to alignment means the middle is hollow.
This testing works only with boards that are too stiff to be affected by gravity.
In building up a table top of very long, narrow boards, each pair will gain stiffness as they are joined. Pairs are then edge-planed together and tested before they get joined into quartets and so forth. Long boards and large work will need clamping, but smaller pairs rely entirely on the grab of the glue.
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