Covenant
A covenant is a group of allied magi. The word "covenant" refers to the oath of cooperation and/or brotherhood they take together, but also (more commonly) to the group itself. In most covenants, the magi live together and share a magical library. Most covenants also have guards (called grogs) and servants (covenfolk).
The covenant plays an important role in most sagas, providing not only a setting for story events but also a common interest for all the magi. It can become a kind of central character in itself: one that outlives any individual player character.
Like characters, covenants can progress (or deteriorate) over the course of a Saga. Ars Magica uses the metaphor of seasons to describe a covenant's development. The core book presents mechanics for describing the covenant, which are expanded in the Covenants supplement.
Under Hermetic Law covenants enjoy certain "communal rights" not accorded to lone magi. Although these are nowhere explicitly noted, they probably include covenant-ownership over shared resources such as raw vis and books, and the ability of Tribunal to enforce the Charter. Different tribunals may also have further rights and duties under their Peripheral Code, and at least some set conditions and requirements to the establishment of a new covenant. It is unclear whether a single magus can count as a "covenant" for the purposes of founding one, but most likely the answer is No as such circumvention of the Code would not be tolerated in most tribunals. Covenants that have been reduced to a single magus through the vagaries of time, however, are an entirely different matter, and may be accepted or rejected depending on precedent and politics.
Magi may reside in the tribunal without belonging to a covenant. If they remain in solitude, they are known as eremites, and this is a respectable choice. Some tribunals also have traditions of hospitality, and a magus can spend decades as a pregrenitor or "wanderer", residing in one covenant after another without being accepted as a member of any. Magi that reside together, functioning effectively as a covenant but without legal sanction, are termed cenobites and are largely despised and even persecuted in those tribunals that restrict the formation of covenants.