When the fingers are curled into a fist, in your head, break each finger down into three dimensional segments to better understand what you are seeing. The red segments are the first segments of the fingers, the blue segments are the second segments. The third segments (the finger tips) aren't visible at all. Notice that the knuckles do not line up straight across the top of the fist. Since the middle finger is the largest finger, the middle finger knuckle is the largest knuckle. In this fist, the second segment of the thumb overlaps the second segment of the index finger. That does happen quite a bit, but not always. Remember - Draw what you see, not what your logical mind tells you what to draw.