In lay-outs, they are static elements because they don't point the reader's eye to any particular direction. On the other hand, triangles, polygons, amoebas and irregular shapes are informal. The square is one of the most common design shapes. Most body copy, framed graphics and some unedited photographs form squares or rectangles. Extreme proportions in rectangles add interest to a layout.
Circles withdraw from other shapes on the page.
They tend to pull the eye into the circle and hold it there. This is called spot value. You can use this spot value to isolate a small-space ad from competing ones or draw readers' eyes into a spot on the page. See the print ad portfolio in the text page 290 and 347 for examples. The triangle's shape pulls the eye away from all else nearby or within.