The aim of this study is to assess the accuracy of the formulae commonly used in our institution for depth of breathing tube placement, and to measure the degree of tube displacement on head movement with different types of tube. Testing the formulae will enable us to be more aware of how frequently inaccurate tube placement may occur. Knowledge of how the different breathing tubes move with head position can help determine the best tube selection to reduce the risk of the tube going in too far or coming out accidentally, for cases were certain head positions are required for surgery.