There is very little difference between under and below. They are both prepositions meaning something is lower or less than something else. Generally under is used more widely than below and if there is some concern as to which word to use the safe choice is under.
The word under can be used when someone or something is under control, under new management or under the command of. These scenarios give one person authority over someone and the other person is under that authority. One item is lower or beneath the control of another. Under is used to mean less than in numerical terms. For example, there were under five students in the class.
There are numerous expressions using the word under before another word:
Under stress – feeling pressured by an emotional situation.
Under someone’s influence – being controlled by someone else.
Under construction – building works that are not completed.
Under a spell – being bewitched or controlled by magic.
Under pressure – feeling pushed into making a decision.
Below is very close in meaning to under and as they are both prepositions they are both used in the same way to describe the position of something or someone. Below is used more often to describe objects on different levels for instance pictures on a wall. One picture may be below the other. Temperatures and water levels are registered below a certain height.
Below is used with words like sea level, surface and poverty line all measures of numerical levels.
Below the surface – just under the top level
Below the poverty line – just under the line of being poor and needy.
Below sea level – land that is below the level of the sea.
The opposite of below is above showing again how this word is linked to distance between things while the opposite of under is over showing the word to be part of a covering. When you go to sleep you lie under the covers.
Both words have some interesting idioms.
Below the Salt: If you are ‘below the salt’ you are being placed in a position below your value. In Medieval times the salt, a valued commodity, was placed in the middle of the table. The head of the table was the highest ranking person so if you were placed lower down at the table below the salt you were less important.