The prefix sub- means “under,” “below,” or “less than.” It attaches to the front of words to show that something is beneath something else, either literally or in rank or degree.

You see sub- everywhere once you start looking for it. A submarine travels under the sea. The subway runs under the streets. Subzero temperatures fall below zero degrees. When you subdivide something, you divide it into smaller parts under the main divisions.

Sometimes sub- indicates a lower rank or position. A substitute teacher fills in for the regular teacher. A subordinate works under someone else's authority. The subcommittee operates below the main committee.

The prefix can also mean “slightly” or “almost.” Subconscious thoughts happen just below your conscious awareness. Something substandard falls slightly below the accepted standard.

Here's a useful trick: when you encounter an unfamiliar word starting with sub-, try thinking “under” or “below” and see if that helps you figure out the meaning. A subtitle sits under the main title. A subterranean cave lies under the ground. Understanding this one little prefix unlocks dozens of words.