When practising eletric, I find it better if I put my guitar on my left leg instead of right. Is there any big disadvantage of doing this or is it just preference?

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I do the same. I usually switch it up depending on what I'm playing and what is more comfortable.
I mostly play on the right leg, but, Each to his own. :P
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practicing on your left leg has one disadvantage, switching from standing to sitting is harder.

Just a little "Spanish Sastle Magic"
well if you think about it, playing classical style, which is on your left leg, is what many consider "proper technique", listen to the guitar world thing for john petrucci on youtube, but it really all boils down to what is more comfortable, because if its not comfortable, theres a good chance you might hurt yourself, and that's never a good thing.
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