Cello Handbook/Holding the Cello
The most fundamental thing about playing the cello, of course, is how to hold it while playing. The cello is held with the sides of the kneecaps resting on the bottom points of each c-bout, with the C-peg resting just behind the left ear. The left hand is used to press the strings, and should be angled so that the thumb is roughly in the middle of the neck and directly behind the middle finger, and the wrist is roughly straight. The feet are not a critical part of playing technique, and while many recommend placing the feet flat on the floor to fascilitate good posture, or simply out of etiquette, many players are known to use their foot as a makeshift metronome to tap out a time. Proper technique will develop in the individual based on personal preference and style, so although there is a correct posture for the cello, don't let that stop you from developing your own style. With certain parts of posture, like foot positioning, if there is no widely accepted "way" to position yourself in that aspect, roll with what works best for you.
The back should be kept straight while playing. Slumping or slouching while playing will not only decrease your ability to manoeuver the bow arm and left hand, but will cause lower back pain if continued for long periods of time. It is recommended that the player sit near the edge of his/her chair while playing. It can help make playing more comfortable and the arms easier to manoeuver about the instrument, as well as prevent the back of the cello from hitting the chair, which can make playing uncomfortable, and can damage the instrument.