Per the breed standard, the skull is square, flat on top, free from wrinkles, cheeks flat, brow abrupt and the stop well defined.
The skull is square. This refers to the plane on top of the head. It’s flat on top, and it forms a square, and is free from wrinkles. The skin should appear tight and not wrinkled like that of a bulldog or Frenchie.
Cheeks flat. If you can grab tissue and muscle with your fingers, that is not a flat cheek. Oftentimes, dogs that have protruding cheeks will give the appearance of a bully head or bully expression.
Brow abrupt. You cannot get the correct abrupt brow in a sunken, almond shaped eye.
The stop. In the Boston terrier, the stop is defined by the region that joins the muzzle. Often, specimens of the breed that do not have a well defined stop will be down-faced.