You have probably been told at least once in your life, “No elbows on the table!” This popular admonition leaves many wondering, “Why not?”
The real answer has nothing to do with your elbows at all, but with your back — that is to say, with your posture. For a long time, proper posture was considered to be extremely important. You stood with your back straight, you sat with your back straight, etc.
If you sit or stand with proper posture, then your elbows automatically relax at your sides. But if you slump forwards, then your elbows drift forwards, too, in order to keep you propped up. So elbows on the table means you’re slumping, which is considered to be bad form (pun intended).
Over time, this posture corollary has been forgotten and people just remember the “No elbows on the table” rule. But really the one stems from the other, so if you’re having trouble keeping your elbows off the table, just try straightening your spine first.