When the thumb is pressed tightly against the side plane of the index finger as shown at top, the thumbnail plane, as well as the top surfaces of the knuckles, will lie at a 90-dcgrcc angle to the top of the hand and the index finger knuckles and fingernail. Note the comraposed angles of the fingernails of thumb and index finger.
The thumb is shown in complete extension in the lower drawing. Notice that at a full 90-degrec angle the thumb fingernail lies fully horizontal w ith the flat surface planes of tte other fingernails. Also, at this point of maximum extension, the thumb and forefinger make a true right angle to each other.
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