Usability Standard 4:
Be Consistent

Users should not have to wonder whether different words, situations, or actions mean the same thing. Follow platform conventions.11

In e-commerce sites, "add to shopping cart" can be easily confused with "buy now." If "add to shopping cart" means "save this item and buy it later" on most sites, it behooves a new site to follow the convention rather than label that same action "buy now" -- or something cutesy like "add to sled."

Furthermore, the site cannot label the same action "buy now" on one page and "add to shopping cart" on another.

Behavioral consistency applies both within a Web site and beyond it to the universe of similar sites; a usable site should use accepted labeling and design conventions and apply them consistently.