Binary Finger Counting: 26 to 31
The thumb represents the value 1, the index is double that, so it's 2, and the middle is double the index, so it's 4, etc. This is key to understanding and fluently counting in binary on your fingers.

To make the number 28 for instance, you have to combine the 16 finger, the 8 finger, and the 4 finger (16+8+4=28).

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Copyright 1995 By: John Selvia

So How Do We Get 1023?
Take a look at this illustration-

Once you've used all 5 fingers on the right hand, start with the pinky of the left hand. The pinky would be double the pinky of the right hand, so it's value would be 32.