Ideally one would like to draw identical lines (or vertices) following
one another and have them join seamlessly, as if a single line had been drawn.
For photons, scalars and fermions there is no difficulty in doing this.
For many styles of gluon this is likewise true. For diagonally-drawn
gluons and those produced in the \CENTRAL style this is not the
case, as illustrated in section 2.10.2. A similar problem ensues when
one wishes to attach a gluon to a vertex constructed from lines drawn in these
styles.