The Vertical-Horizontal Illusion refers to the human tendency to overestimate or underestimate the length of a vertical (up and down) line as opposed to a horizontal (right to left) line when these lines bisect (cross) one another. The human eye tends to interpret horizontal lines as being longer than vertical lines, even when the lines are the same length. Interestingly, people accustomed to urban landscapes are more susceptible to this illusion than those who are more accustomed to open landscapes.
You can see an example here (not on AlleyDog.com): Click here to see an example
See also: Mueller-Lyer Illusion