The Russian Orthodox Church must choose a new leader within six months of Friday's death of Patriarch Alexiy II.
Following are extracts from Russian Orthodox Church's charter and some facts about how Patriarchs are selected and the procedure for doing so:
The Holy Synod or the ruling body of the Russian Orthodox Church has to convene immediately and elect a Patriarchal Locum Tenens or an interim Patriarch from among its six permanent and six invited members.
It will be chaired by the oldest of its metropolitans or senior bishops, Metropolitan Vladimir of St Petersburg and Ladoga.
Church officials say the Metropolitan of Krutitsy and Kolomna, Juvenali, may well become the Patriarchal Locum Tenens when the Holy Synod is convened on Saturday. That group will also decide on the funeral ceremony for Alexiy.
The Patriarchal Locum Tenens has the same rights as the Patriarch apart from the right of giving titles and decorations to bishops.
The Holy Synod and the Patriarchal Locum Tenens must then convene an archbishops' synod. This body, consisting of around 200 members, will then convene a wider synod of all senior bishops of the Russian Orthodox Church worldwide and elect a new Patriarch within six months of the death of the Patriarch.
The big synod elects the Patriarch behind closed doors.
Only archbishops older than 40 can become candidates.
The big Synod must approve the procedure to select the new Patriarch.
Alexiy was elected by secret ballot in 1990 and faced competition from two other candidates.
During Soviet times, three patriarchs have been elected by open ballots with no alternative candidates and with prior approval from the state.
In 1917, upon the re-establishment of the Patriarchate after a nearly 200-year-suppression, Patriarch Tikhon was chosen by drawing lots after he and two other candidates were pre-approved by the big Synod.
This method is intended to allow the Holy Spirit to guide the choice.
The inauguration of the new patriarch -- the enthronement -- will take place a few days later in one of Moscow's principal cathedrals, most likely Christ the Saviour, which was rebuilt in the 1990s after its demolition by Stalin.