Although Horatio is the ultimate leader of the CSI team in Miami, Calleigh has always been his unofficial second-in-command. Both Horatio and Calleigh are totally committed not only to justice for the victims of the crimes they investigate, but to each other and their team. Time and again Horatio has counted on Calleigh for help and she never hesitates to give it - always coming through for him. By the same token, Horatio is incredibly protective of Calleigh - though he never underestimates or diminishes her abilites as a cop and a CSI. Both characters have a tendency to internalize their emotions and rarely let other's in, but their trust in each other is absolute and unshakable. They are also in tune with each other - often to the point of being able to communicate without words. Out of all of the CSIs, Calleigh and Horatio seem to know each other the best. They are a great pair, and Horatio would never disappoint, disrespect or hurt Calleigh like others have done in the past.
This pairing is most often referred to as DuCaine using their last names together, but it is also referred to as Lonely Road.
This ship name got its namesake from episode 01x17 "Simple Man".
Det. John Hagen: All I'm saying is, that's one hell of a lonely road
Calleigh: I know. That's why I'm walkin' it with him.
Existence of DuCaine through the seasons
Calleigh: We're much more fanciful down here. Aren't we, Horatio?
Horatio: I think that's a fair description
from 00x00 "Cross Jurisdiction"