The biggest problem with teaching yourself (guitar or anything else) is that you don't know what you don't know.
I've been teaching guitar for a long time, and I still end up being surprised by some of the unorthodox moves beginners create. But these are some of the most common mistakes I see:
- holding the guitar with the peghead too low. You want your fretting hand fingers to be parallel to the frets, because that gives you maximum reach. If the neck is horizontal, or even worse, angled down, you won't have very good reach, and it will put strain on your wrist.
- tipping the guitar so you can see where you're putting your fingers. Doing that bends the wrist at an awkward angle. For minimum strain, you want the fretting hand wrist to be straight - you should be able to draw a straight line from your elbow to the knuckles at the base of your fingers with no detours along the way.
- Hooking the thumb over the neck. This brings the palm of your hand too close to the guitar, making it difficult to fret cleanly and play quickly.
With these and other most errors, there are exceptions. Guitars come in few sizes, people in many. Folks with large hands/long fingers can get away with bad technique longer than average folks. Certain techniques, like large bends will require changing the hand position to create leverage (most often by hooking the thumb). So they aren't absolutes - but they are best practices.
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