Doug,
There are two uses of the word "tiller." The one Ted describes is "to tiller" a bow. This is part of the process of making a bow and getting the limbs bending evenly and correctly.
The second use of the word is the "tiller of the bow." This is what reddogge is describing. But to take the explanation farther... When you have a completed bow, you can measure the distance from both the upper fade out of the riser and the lower fadeout to the string(perpendicular). A bow is said to have a positive tiller if the upper measurement is (usually) 1/8 to 1/4 inch greater than the lower measurement. This usually is the tiller you want for split finger shooting.
If the measurement is the same, then you have "even" tiller and that is most often what 3 under shooters use.
When a bowyer builds a bow, during his tillering process, he gets the limbs bending nicely AND sets the final tiller measurements.
Arne