Why is the sphygmomanometer kept at level of your heart while measuring blood pressure?

The sphygmomanometer cuff is placed at the same level as your heart (ie the fourth intercostal space) to ensure that there is no hydrostatic pressure difference (force exerted by a fluid or gas at rest due to gravity) between the cuff and your heart (which would skew the results). Keeping the sphygmomanometer at the heart's level eliminates the effect of gravity on the blood pressure measurement, ensuring an accurate reading.