The Electronic Entertainment Expo (E3) is a pretty big event in the video game industry, giving publishers and developers the opportunity to showcase upcoming games and make big announcements. This year, Microsoft stunned everyone by announcing that they were making the XBOX One console backwards compatible. This is a huge announcement, and one that will likely put Sony on the defensive.

What is backwards compatibility? Simply put, it means that players with an XBOX One will be able to play at least some of their old XBOX 360 games on their newer console. Backwards compatibility used to be assumed with new consoles. The Playstation 2 and 3 played older Playstation games, and the XBOX 360 played a large selection of original XBOX titles. But neither the XBOX One nor Playstation 4 offered backwards compatibility when they launched.

Although both consoles have sold well, there are fans who have held back because they want to continue playing games on their existing consoles without having to give them up or find some way to connect yet another device to their TV. Backwards compatibility allows players to continue using at least some of their existing library of games without having to switch machines. That’s a big draw for some people. It also means that they can sell more games for the XBOX One, which currently has a much smaller library of games than does the XBOX 360.

The system isn’t in place yet, and in fact it won’t be until this fall that the majority of XBOX One players will have access to backwards compatibility. When it launches, though, it will be free, offered as part of the regular updates to the XBOX One operating system. The goal is to have at least 100 XBOX 360 titles available to play on XBOX One by then, with more titles coming out over time.