Archontology.org: Conventions

Common abbreviations
[1]   Footnote (if digital inside the brackets is not in date format)
O.S.   Date in Old Style (Julian) calendar. Example: 17 Apr 1584 O.S.
N.S.   Date in New Style (Gregorian) calendar. Example: 8 Nov 1917 N.S.
(or... )   Used in dates, such as "6 (or 7) Sep 1724", to emphasize the ambiguity when sources significantly contradict to each other and/or provide insufficient evidence for precise dating.
c.   Circa is used when the dates of events are approximately known. Example: c. 14 Dec 1542
Special Signs
/   Slash is used to separate the dates given both in Old Style (Julian) and New Style (Gregorian) calendars for the nations which continued to use the Julian calendar after its original introduction in some European states on 15 Oct 1582 N.S. In some cases a New Style date is given in [square brackets]. If a date is not paired, it should be assumed that it is calculated according to the calendar which was in official/public use in geographical zone where and when the event occurred. Examples: 19/29 Feb 1644, 19 Feb/2 Mar 1723, 23 Dec 1584/2 Jan 1585, 11 [22] Feb 1732.
  Iteration sign is used to stress that the name on a list appears at least second time or more frequently. The sign consistently appears if the name had appeared on an earlier in the same category. Different categories, for instance "Heads of State" and "Heads of Government", do not share such cross-referencing.
Format of Names and Styles
Algeria :: al-Jazā’ir   If found in headings, the name of the nation in English is followed by its equivalent(s) in official language(s)
Dei gratia Rex Scotorum = By the grace of God, King of the Scots   If found in styles, stresses the equality of use, when two or more languages are in official use
Carlos III (Carlos José Francisco)   This format is used to provide a monarch's original, personal or baptismal name before accession when a link to biographical entry is missing.
William II (= William Henry, Prince of Orange)   This format is used to indicate that a person appears on a list/on lists under different names (normally followed by the iteration sign (x), see above)