The Smashing Book - Typography: Rules, Guidelines and Common Mistake (part 2 of 4) | Smashing Magazine

At its core, hierarchy supports legibility by providing clear visual clues about the structure and organization of the text. Hierarchy defines how content is to be read through. It shows the user where to start reading and where to break. It differentiates headers from body text. Although text box color can be used to give headers and body text contrast, hierarchy shows the difference in size between these elements. Hierarchy plays a huge part in making a layout scannable and is an important technique for making Web typography readable.

AIGA achieves a beautiful, clean and intuitive hierarchy through a subtle use of color, font size and leading.