Color charge is conserved, but the book-keeping involved in this is more complicated than just adding up the charges, as is done in quantum electrodynamics. One simple way of doing this is to look at the interaction vertex in QCD and replace it by a color-line representation. The meaning is the following. Let represent the -th component of a quark field (loosely called the -th color). The "color" of a gluon is similarly given by which corresponds to the particular Gell-Mann matrix it is associated with. This matrix has indices and . These are the "color labels" on the gluon. At the interaction vertex one has . The color-line representation tracks these indices. Color charge conservation means that the ends of these color-lines must be either in the initial or final state, equivalently, that no lines break in the middle of a diagram.