Top weight/bottom weight, positive/negative side weight, and finger/thumb weight are all differences in the weight of two halves of the ball. For example, on an undrilled ball, place the true (not necessarily the marked, that's a discussion for another thread) center of gravity (cg) dead center on the top of the ball. Now, come down to the equator of the ball and saw it it half. If the top half weighs more than the bottom half, the ball has top weight equal to the difference in the weights. If the bottom half weighs more than the top half, it has bottom weight. So you don't have top and bottom weight in the same ball (unless you're a mathematician, then the top and bottom weights are the same absolute value but with different signs). Of course, you don't actually cut the ball in half, you draw a line and use a balance (the most popular type is called a Kaufman balance) that compares the weight of the two halves and gives you the difference.
For a drilled ball, the grip center (gc) replaces the cg at the top of the ball to determine where to draw the line for top/bottom weight. A line from the center of the bridge through the center of the thumb hole and continued around the ball (this is called the grip centerline) determines the two halves for positive and negative side weight. Again, you don't have both but one or the other. And a line perpendicular to the grip centerline through the gc (this line is called the grip midline) and continued all the way around the ball determines the two halves for finger/thumb weight. I hope this helps clear things up for you. -- JohnP
