The relationship between pressure and depth is exploited in manometers (or barometers) that measure pressure. Pressures are measured by comparing the pressure at one end of a container with a known pressure at the other end.
A standard mercury barometer to measure atmospheric pressure is a tube with one end sealed. The sealed end is close to zero pressure, while the other end is open to the atmosphere. The pressure difference between the two ends of the tube can maintain a column of fluid in the tube, with the height of the column being proportional to the pressure difference. If the closed end has negligible pressure, the height of the column is proportional to atmospheric pressure being measured.
Mercury is commonly used in barometers because its high density means the height of the column can be a reasonable size to measure atmospheric pressure. A barometer using water, for instance, would need to be 13.6 times taller than a mercury barometer to obtain the same pressure difference. This is because mercury is 13.6 times more dense than water.
In an open-tube manometer, one end of the tube is open to the atmosphere, and is thus at atmospheric pressure. The other end is connected to a region where the pressure is to be measured. Again, if there is a difference in pressure between the two ends of the tube, a column of fluid can be supported in the tube, with the height of the column being proportional to the pressure difference.
The actual pressure is known as the absolute pressure; the pressure difference between the absolute pressure and atmospheric pressure is called the gauge pressure. Many pressure gauges give only the gauge pressure.