Dobut Regarding Even and Odd Mode Velocity

Helo Friends,
In coupled line filter harmonics occur because of the disparity of velocities in EVEN and ODD mode(Even Mode velocity is more and ODD mode velocity is less).It is written in books that to decrease the dismilarity in both modes, either hole is made or line length is increased for even mode.I need elaborate details on these. Any body can clarify my doubts or send some soft copy on modes.
Regards
Bibhu