Similarity

Similarity: objects that look similar to each other are perceived as a group.

In this design, there are a number of different triangles, but you likely see them as a whole logo or a bird's head and a sun:

You can add visual interest to your page and draw viewer's attention to part of a design by making one part different from the rest. Here your eye is drawn to the person on the far right because that figure is different:

Google uses similarity to show the status of the system by making the "o" red for the page the user is on, and uses the yellow "o" indicating pages of results present:

You can display the structure of information using the principle of similarity. For example, place key terms flush with the left margin and put them in bold.

References: First two images, Google example