180 Degree Rule

180 degree rule 

This is similar to shot reverse shot and is commonly used in conversations between two characters to make sure their dialogue runs smoothly. A straight line called the axis is often imagined running between and in front of the two characters to make sure that when you film it the camera doesn't go over the line. The line enables the audience to orient themselves with the position of the characters in the scene. If the camera did go over the line then the shot wouldn't work properly and you would loose the concept of the characters conversation and where they were. If the camera stays on the same side of the line the relationship between the two characters will be consistent throughout the shots and this makes sure the first character is always frame right of the second character, who is then frame left of the first. This rule enables the audience to connect with the unseen movement of the characters. 

This is an example of the 180 degree rule from the hunger games.

This is a diagram of the 180 degree rule.