Beginning with the upcoming core set, we will no longer be using "Nth Edition" to identify core sets. Instead, we will use a year designator. The July 17, 2009 release will be the Magic: The Gathering 2010 core set—Magic 2010 for short—to reflect the upcoming year.
Beginning with the Magic 2010 core set, all Magic core sets will be releasing annually in July instead of biannually. The core set currently scheduled for release in July 2010 will follow the same pattern and will be the Magic 2011 core set.
Unlike previous core sets, which were all reprints, the Magic 2010 core set will have approximately 50 percent brand-new cards. Following in the footsteps of Tenth Edition, all core sets going forward will be black-bordered. Also of note is that planeswalkers will appear in the core set for the first time starting with Magic 2010. The set size and rarity mix will still follow our new guidelines as announced last year. Magic 2010 contains 249 cards, including 15 mythic rare cards.
Since the core set now contains new cards, there will be Prerelease events and Launch Parties just like any other expansion set. Going forward, core sets will also be an integral part of high-level organized play. The Magic 2010 core set will be used in Limited-format Grand Prix events this year.
The Magic 2010 core set will become legal in the Standard format immediately on release. At that time, Tenth Edition will rotate out of Standard. Beginning with the Magic 2010 core set, each core set will be legal in Standard until October of the following year (each Magic core set will be legal in the Standard format for approximately 15 months), meaning that Standard rotates only once each year.
For more information about these changes, see Aaron Forsythe's feature article.