This is the glove from the book Le Gant, it was constructed in 1640:

Here are the pieces also from Le Gant that closest resemble a finished product of that glove in style. fourchettes Hand Thumb The Cuff

Please realize that I did not change the lines of the original drawings. I only cleaned up the interiors and straightened the lines as needed. You will notice that the thumb is not symmetrical, this is important. At first I thought it was just an irregularity on the part of the drafter. But that little voice in the back of my head said to check the other patterns thumb pieces. They were also sloped the same way. The modern pattern no longer has this slope because they were mass produced in the last century and now this one by using stamps to cut out the leather or cloth pieces.

Uniformity was important in cutting costs. There was less waste this way. There are documents where manufactures boasted that they can now get 7 pairs of gloves out of a section of hide where they used to be only able to get 3 or 4 at the most.

The comparison that best explains the look of the thumb piece is the sleeve part of a garment. If you look at a sleeve pattern you will see that there are two different lines in the curvature of where it connects to the body of a garment. The thumb is also like an arm, it is fitted with a "sleeve" to the body of the glove. If you look at the fourchettes you will see a similar phenomena. Here it compensates for the depth between each of the fingers. This is where your fitting will be precisely to your hand. The cuts of the back of the hand are longer than for the palm side of the hand. This is so that they can take in account for the diagonal line between your fingers at the fold.