Fifth Base is a unique rule to Blaseball that places an additional base on the field which players are required to run through before reaching home plate.
On the Season 7 Election Ballot, Fifth Base was offered to the league as a decree. At that time, Fifth Base was not selected, with the vote going to Bless Off instead.
In the Season 9 Election Ballot, Fifth Base Bubble was offered to the league as a blessing, promising to protect the subleague of the team that won the blessing. The Baltimore Crabs won the blessing, protecting Wild High, but exposing Mild High, Mild Low, and Wild Low to Fifth Base for the duration of Season 10.
When Fifth Base is active, an extra plate is placed behind third plate, opposite from home plate. The Commissioner tweeted a diagram of the path players must run to reach home plate. Players run through first, second, and third plates as normal, then they run away from home plate to the fourth plate, before running back past third to reach home.
The Blaseball simulation appears to account for the extra distance running to the fourth plate, with fourth plate being easier to steal, and home plate being harder to steal.
- BLASEBALL COMMISSIONER [@blaseball] (12 Oct 2020). (Tweet)- via Twitter.