When we discuss key issues, they’re usually broken into two categories: major and minor. The easiest way to tell the difference between them is price but those higher prices are there for a reason.
Major keys are issues with a large impact not only on that particular series but on the industry and collecting. In this case, the bulk of major keys deal with first appearances, and they come with hefty price tags. These aren’t just any first appearances (sorry, Hydro-Man). They are the debuts and major moments for significant characters with massive followings. In many cases, these issues are hard to come by, which makes them even more collectible.