South-east rule

All blocks are 16 pixels in size (in all dimensions). It appears that the north and east edges of blocks is where they 'start'.

The South-West rule is likely due to the coordinates (viewable by pressing F3). South is positive X and west is positive Z.

This has many effects in game:
 * When placing minecart tracks it will favor linking to the North or East edges.
 * Minecarts will favor South or West motion at a "T" due to the fact they favor 'starting' at the North or East edge.
 * The North and East faces of a block on fire will burn a player who touches those sides.
 * The Northeast corner of a block that has blocks to the North and East of it will effect its properties to a block (or player) touching that Northeast corner. All other corners are 'safe' since they have a South and/or West edge buffer.
 * Lava touching water does not create cobblestone, water trying to enter a lava block does.
 * A player, boat, or other NPC touching the Northeast corner of a water block can swim even if blocked on the North and East sides.