Build Your Own Block is a casual constructed Magic: The Gathering format that allows players to create their own "Block" of sets as a Block Constructed environment.
Each player selects the first set from any block (such as Lorwyn or Shards of Alara), the second set from any block (such as Guildpact or Morningtide), and the third set from any block (such as Scourge or Future Sight). That player then builds a Constructed deck using only cards from those three sets. He or she will then play against opponents using the same deckbuilding constraints, but not necessarily the same sets! 
Build Your Own Block is a fun way to find out how the tribes of Lorwyn would have fared when combined with the tribes of Legions and Scourge, or how an artifact set like Mirrodin might have interacted with Urza's Legacy.