What is the difference between "under" and "below"?

Several readers have recently asked about the difference between the prepositions under and below, which are often interchangeable. 

Here are four facts about under and below that will help you understand the difference between these words and use them correctly:

1. Under is used much more widely. When you’re not sure which word to use, use under. It is more likely to be correct. 

2. All the common uses of below are related to the idea of “lower or less than,” as in these examples:

• The sun disappeared below the horizon. 
• Temperatures were below normal all week. 
• The game is suitable for children below the age of 10. 
• There are many more fish below the surface of the water.

3. Under can have the same meaning as below, as in these examples:

• We sat under a tree and rested. (Or, We sat below a tree and rested.)
• Draw a line under each word you don't know. (Or, Draw a line below each word you don't know.)

   but under can also mean “guided or controlled by,” as in: 

• The cafe is under new management. 
• The army captured three forts while under the general's command. 
• Under the terms of the lease, rent will be due on the first of each month.

4. Under is often used in expressions in which it has a special meaning. Look at the most common collocations with  under to see this more clearly:

• Hudson tried to get her weight under control with drastic diets. [under control=able to be managed successfully]
• Under normal circumstances, Steinmark might have wished to spend more time in Los Angeles [under normal circumstances=in a normal situation]
• You have to be able to think under pressure, [under pressure=in a stressful situation]
• Under the law, hospitals have to treat any patient who needs emergency medical attention. [under the law=according to the law]

I hope this helps.