Elements in a design should line up for cohesiveness. This can be arranged in terms of rows, columns or even angles.
Source: Lidwell, William. Universal Principles of Design: 125 Ways to Enhance Usability, Influence Perception, Increase Appeal, Make Better Design Decisions, and Teach through Design. Rockport, 2010.
Source Example
Online Example
Real Life Example
Interesting one. I feel like there are case in which this might make designs limiting but in a practical sense. It took me some thinking to get that you we were aligning your pillows. 🙂 I wasn’t sure how it related directly but I know the in real life examples are sometimes a challenge. I had to dig to find some of mine.
I am comparing Hipchat, Facebook’s Messenger.com and Slack messaging, I am realizing that they all use three column alignment design to display different information in each panes. Or Instagram! They whole visual design of their application involves alignment of same sizes photo square to create a cohesive experience.