The trot is a jumped gait, symetric, with two beats, by diagonal bipeds.
It is the gait with which the dog has got more endurance, is is why it is usual when hunting. It is also the gait used by the dogshow judges to check the limb dynamic of a dog.
The landing of a hindlimb is done at the same time as the landing of the forelimb on the opposite side, which gives the following beats:
right hindlimb + left forelimb -> left hindlimb + right forelimb
In fact, the hindlimb lands slightly before the forelimb, but it is difficult to see for a human eye.
At normal speed, the dog has always two feet on the ground.
When the speed increases, a suspension phase occurs between each thrust phases.
The old braque du Bourbonnais was said to be a trotter. During the recreation of the breed in the 70's, we couldn't find back this caracteristic.
Yet, I have seen in my youth a half-blood we owned and who had this sustained trot described in the old days. It was often using it and with this gait it had the same speed a dog of the same size would have had with a slow gallop. Alas it was sterile, and it couldn't pass its gait to today Bourbonnais.
One can then ask himself the question: what makes a dog a trotter? Here is where I am in my thoughts on the subject:
It seems that the trotter horses generally have a straight shoulder, i.e. a quite vertical scapula. This features, preventing them from reaching filed far ahead with their forelimbs, seems to facilitate the trot compared to the gallop.
It is a subject which can lead to controversy, because some people would like a Bourbonnais with a very short body, like the Brittany spaniel. It seems to me that this caracteristic is not desirable for a trotter dog, and that the old Bourbonnais were not very short.
It is easy to see on the pictures above, that at the first and third step of slow trot (first, third and fourth of fast trot), the forelimbs and hindlimbs can interfere with each other if the body is too short compared to the limbs. It is why the Brittany spaniel is not known for its trot, while the German shepherd, with its long body, is a renown trotter.