Many people in the natural horsemanship world get these two terms confused, so I thought I would write a short post detailing the difference, and what each one means in terms of training and fitness.
What it is: Self carriage is the way the horse naturally elevates his body and holds himself. Ideally self carriage shows a horse with a rounded back, an elevated head and a relaxed poll. His movements should be loose and free flowing. This should be achieved with little to no rider influence.
What it looks like:
What it means: A horse that can achieve self carriage is in good physical condition, and is also very comfortable with his or her surroundings. If a horse isn't comfortable, they will often become tense through the poll and tail, and will not track up properly behind.
What it is: Collection is a version of self carriage, and is essentially an advanced one. In collection, a horse should maintain self carriage but should bring his weight toward his haunches. It is imperitive that the haunches remain the source of power, but that the horse sends that energy up rather than out.
What it looks like:
What it means: Collection shows a horse that is in excellent physical condition, and is listening and supple to his rider and partner.
What They Are Not
Collection and self carriage cannot be properly achieved through rein contact alone. They are not "see-sawing on the reins until the head goes down", because the head position is a symptom of proper positioning of the body and more specifically the spine. COLLECTION IS NOT BELOW HORIZONTAL WITH THE NECK OR BEHIND VERTICAL WITH THE NOSE. Period. That is not proper collection. Collection also cannot happen when the horse's stride is extended. If the stride is extended, it is self carriage.
I hope this helped clarify a few terms I see misused a lot. Friendly discussion is always welcome on this post and the corresponding picture on Instagram (@dappled_grey), but hateful comments will be deleted. Thank you!