Played in the town centre of Kirkwall, the two sides are the Uppies and the Doonies, or more correctly, "Up-the-Gates" and "Doon-the-Gates" from Norn gata (path or road), although it is also common in Scots.
Scant information is available about the early history but some form of mass football appears to have been practised throughout Scotland and England for at least three centuries.
There is no restriction on the lower age limit and small boys of sometimes as young as 5 years old can be seen around the edges, making their first tentative steps into the game.
Much exciting surging and turning play often occurs on this wider part of the street, which can frequently determine the outcome.
[6] Immediately after the end of World War II there was a movement to establish a Ba' game for women in Kirkwall.
[7] The Doonies' goal is the sea, normally within the Basin of the Harbour, but so long as it is immersed in the salt water of Kirkwall Bay, the Ba' has gone doon.
The Uppies must round the Lang, or Mackinson's corner at the junction of Main Street with New Scapa Road, opposite the Catholic Church, which was the site of the old town gates in bygone times.
(The last remains of the town gates were removed in the 1950s, leaving the Uppie goal as the gable end of a house on Mackinson's Corner.)
Once the Ba' has been goaled Up or Doon, lengthy argument often ensues among players on the winning side before a popular individual winner is acclaimed.
To have any chance of this honour, a player generally has to have played consistently well for a long period of time (usually around 20 years minimum).
Ba' winners range from outstanding players in their early thirties to veterans in their mid to late forties.
For non-Orcadians or ferryloupers (incomers) or indeed anyone with no family history in the game, and often people from the isles or rural areas, their side is determined by the route taken on their first arrival in Kirkwall.
The scrum will collapse often during the game, at which point players are generally very good at ceasing play and aiding teammate and opponent alike to regain their feet.