1. To stand facing someone or something in a row in opposition. The football team lined up against its opponents, eager for the ref to blow the whistle. The two rival gangs lined up against each other, the tension so thick you could cut it with a knife.

2. To stand or organize in solidarity against a person, group, or organization. The traditionally hostile political factions are joining together to line up against the terrorist group.

3. To organize people in solidarity against a person, group, or organization. In this usage, a noun or pronoun can be used before or after "up." The controversial senator has been whipping people into a frenzy with his political rhetoric, lining up voters from around the state against his opponents. Instead of lining teachers and parents up against one another, we should be trying to find solutions that involve and hold accountable both sides.

4. To stand in a row against some surface, especially a wall. OK, children, line up against the wall in alphabetical order. The actors lined up against the back of the stage and then stepped forward to take a bow.

5. To cause a group of people or things to form or get into a row against some surface, especially a wall. In this usage, a noun or pronoun can be used before or after "up." The coach lined us up against the back of the court and made us do sprints. Line up these boxes against the wall over there so we can sort through them later.