The 180º Rule is often used when characters are engaging in conversation. An imaginary line called an axis connects the two characters, by keeping the camera on just one side of the line for every shot in a scene, one character is always to the right of the other and vice versa. If the camera crosses over the axis, it causes confusion for the audiences as it becomes unclear as to where the characters are stood and whether they are moving. The object or characters that are being filmed must me in the centre of the camera rotation and the camera must always face what is being filmed.
Megan Jackson's AS Media Studies Blog 