excerpts from USEF Rulebook Chapter DR
DR101: Object and General Principles of Dressage
6: The head should remain in a steady position, as a rule slightly in front of the vertical, with a supple poll as the highest point of the neck, and no resistance should be offered to the rider.
DR102: The Halt
At the halt the horse should stand attentive, engaged, motionless, straight and square with the weight evenly distributed over all four legs. The neck should be raised with the poll as the highest point and the head slightly in front of the vertical.
DR103: The Walk
The horse, remains “on the bit”, moves resolutely forward, with its neck raised and arched and showing a clear self-carriage. The head approaches the vertical position and a light contact is maintained with the mouth.
The rider allows the horse to stretch out the head and neck (forward and downwards) without losing contact with the mouth and control of the poll. The nose must be clearly in front of the vertical.