This month, researchers made official something that typeface designers have long known: that horizontal lines appear thicker than vertical ones. At left, a square made from equally thick strokes; at right, the one that feels equally weighted, its vertical strokes nearly 7% thicker than the horizontals. This phenomenon, central to typeface design, has implications for the design of logos, interfaces, diagrams, and wayfinding systems, indeed anywhere a reader is likely to encounter a box, an arrow, or a line.
Published in the journal Vision, this peer-reviewed paper confirms that most people overestimate the thickness of horizontal lines. This is the very optical illusion for which type designers compensate by lightening the crossbar of a sans serif H, an adjustment that’s easily revealed by looking at a letter sideways. When rotated, the evenly-weighted Gotham is revealed to have thicker verticals than horizontals; try the same in Ideal Sans, a typeface designed to push against the boundaries of what we normally notice when we read, and it becomes clear how little we actually see of what is there.