Your thumbs are special in at least two important respects:
- Your thumbs are the most centrally-located fingers, most directly in front of you in your field of vision.
- Your thumbs, in combination with any of your other fingers, allow you the maximum reach within the span of one hand.
Implications: Whenever you play notes outside the middle registers of the keyboard, you can locate those notes without having to turn your head to look. Simply pay visual or tactile attention to where your thumbs are, then use your fingertips and proprioception to effectively give you an extra octave on either side without looking!