AKC Breed Standard
The GDCA also maintains an Illustrated Standard that describes all aspects of the GDCA Breed Standard.
The breed standard describes the characteristics of Great Danes that are desirable or undesirable. Breeders are responsible for preserving and improving the characteristics of the breed. Many of those characteristics are judged in a conformation dog show - it is the reason we have dog shows (for conformation).
Show judges are the experts. Judges in conformation shows evaluate the characteristics of each dog, following the breed standard, with each judge deciding how to balance the breed characteristics to rate each dog, given the characteristics that they can observe quickly in the show ring. Different judges balance those breed characteristics slightly differently - it is the way the breed experts express their opinions about which characteristics are most important.
But after all the judging, shows, wins and titles, breeders still have the final responsibility of achieving the best balance between characteristics in each breeding. Breeders, having many years (often generations) of experience evaluating their breed, try to choose breeding pairs that will produce the best balance of physical characteristics, temperament and health in their litters. The breed standard is the negotiated agreement from breeders and breed experts describing the characteristics that define the breed.
The GDCA publishes and occasionally updates the Great Dane Breed Standard. An update to the standard has been proposed, and is in the process of being accepted and approved now. This update to the standard includes mantle merle acceptance, and changes in back length, minimum height, dentition, and brindle masking. See the GDCA website and the latest GDCA Bulletins (e.g. 2017 Q2 Bulletin, page 3) for a description and the latest news on the update.
Our club, the Great Dane Club of Central North Carolina, follows, endorses and promotes the GDCA Breed Standard. As an affiliate club to the GDCA, we are involved in the process of all updates, including proposal, debate, review and voting.