Its often crossed my mind this question of seniorty.
If length of service is a deciding factor.....what if the person involved is a complete d1ckhead / poor at his (or her) job etc. Should said person automatically be considered above others purely for their length of service. When perhaps others who give that little bit extra but with less years service stand out above the 'oldtimers'
No idea about the legalities surounding this but has ocasionally been one for my 'idle curiosity' box.