Combing - also known as 'feathering' or 'line twitter' occurs with interlaced video sources and affects vertical lines during camera movement. Interlaced images are made up of two fields which make up each frame. The first field contains the odd numbered scan lines (1,3,5,7,9 etc.), the second field contains the even lines. In a panning where a given object is moving on the screen, it will have moved a small distance between field one and field two in effect meaning that the field one "half" object will be slightly out of line with the field two "half object" which causes the object to appear to shimmer. This is due to the "straight" line not actually being straight but being jagged.