The earliest ordinations described in Buddhist tradition were far simpler than the procedure practiced today: early sources describe the Buddha personally admitting new monks with a short, direct invitation, traditionally rendered as come, monk, with no committee, boundary, or extended formal process required beyond the Buddha's own spoken acceptance of the candidate. As the monastic community grew well beyond what the Buddha could personally oversee, this informal method gave way to the formal procedure preserved in the Vinaya and still used today, requiring a quorum of already ordained monks acting together, a formal motion proposed and confirmed through a specific ritual question and silence procedure, and a consecrated boundary, the sima, marking out the specific physical space within which a valid ordination must take place.
The sima requirement is not a minor technicality; a valid sima must be properly established and its boundaries formally agreed by the local monastic community before any ordination held within it counts as valid, which is why disputes over whether a given sima was properly consecrated have historically had the same kind of practical consequences for the validity of ordinations performed there that disputes over a lineage's unbroken transmission have for individual monks and nuns ordained through it. This shift, from a single teacher's personal acceptance to a formal communal procedure bound by consecrated space and a quorum of witnesses, reflects the wider pattern by which the earliest, most informal phase of many religious movements gives way to codified institutional procedure as the community outgrows what its founder alone can personally administer.