Bring Your Own Block

Bring Your Own Block, or 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. 
==Description==
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. They will then play against opponents using the same deckbuilding constraints, but not necessarily the same sets! 
Bring 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. 
===Invitational use===
As an example, Magic Invitational Fall 2000 used this format and required that the three sets picked be:
*One of Ice Age, Mirage, Tempest, Urza's Saga, Mercadian Masques, or Invasion
*One of HomelandsThis was before the release of Coldsnap and therefore Homelands was still considered part of Ice Age block, Visions, Stronghold, Urza's Legacy, or NemesisAt this time, Planeshift and Apocalypse had not been released yet, hence they were not options.
*One of Alliances, Weatherlight, Exodus, Urza's Destiny, or Prophecy
This style somewhat breaks down in the later periods of the Block Model when some second and third sets became full-size expansions like the first sets. Later users of BYOB are more likely to choose the more permissive model of "any three expansions". It is also possible to stipulate that you may not pick all three sets from the same block, or the even stronger condition "no two sets from the same block".
==References==