ContinuityEditing(also called time-scheme) is consistency of the characteristics of persons, plot,objects, places and events seen by the reader or viewer over some period of time. There are different continuity techniques:
Match-on-action: A shot that emphasises continuity of space and time by matching the action of the preceding shot with the continuation of the action.(For example a shot of a door opening after a shot of a close up of a character’s hand turning a door handle). This is important in helping the audience to follow exactly what's going on.
Shot-reverse-shot: Where one character is shown to look at another character and other way round. Since the characters are shown facing opposite directions the audience assume they are looking at each other.
180 degree rule (crossing the line): The idea that a camera should be placed somewhere inside 180 degrees on a particular side of the invisible line of a shot that contains two people filmed in sequence. If the camera crosses the line, confusion results for the film viewers, because it makes it look like the people are switching places (two character should always have that left and right relationship with each other).