Split counterlungs are used when there are two counterlungs, one for the inhaled gas and one for the exhaled gas. They are flexible bags and must have enough volume to contain a full inhalation or exhalation. They can be mounted on the front, back or over the shoulders of the diver, their position affects the breathing resistance at various attitudes. A front mounted counterlungs makes inhalation easier than exhalation in a normally swimming diver. With a back mounted counterlungs exhalation is easier due to hydrostatic pressure. The ideal position for a counter lung is when it is balanced to the centroid, this point is about 19 cm below and 7 cm behind the sternal-notch in the upright position, in the average person. With twin shoulder mounted counter lungs there are several advantages. On inhalation the upstream counter lung collapses and causes gas to move from the downstream counter lung due to hydrostatic pressure. This helps the gas move through the CO2 adsorbent making the work of breathing less. Conversely on exhalation the downstream counter lung inflates, this causes a pressure differential with the inhale counter lung which draws the gas through the breathing loop ready for the next inhalation. Slit counterlungs are not always "over the shoulders". The dolphin has split counterlungs but not over the shoulder.