Structure and navigation must support each other and integrate with search and across sub-sites. Complexity, inconsistency, hidden options, and clumsy UI mechanics prevent users from finding what they need. Bad information architecture causes the majority of outright user failures and isn't improving at the rate of other Web usability issues. To determine why, I've identified 10 long-term sore thumbs that together cost websites billions of dollars each year. [...] I divided the following list of worst IA mistakes into two parts, which corresponds to how we partition the materials across our 2-day IA course: structure on Day 1 and navigation on Day 2 . Of course, you need to get both right, but they're essentially two different design levels: The invisible way the site is structured and the visible way users understand and manage that structure.