The fingers on both hands are assigned numbers for ease of communication when learning a piece of music.
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T - thumb (the thumb isn’t ordinarily used in the fretting hand, so it doesn't receive a number)
1 - Index
2 - Middle
3 - Ring
4 - Pinky
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(LINK) For more, refer to Evan Taucher’s Pickup Master Class: https://pickupmusic.com/learn/evan-taucher/