Master Mason is the third and highest of the three Craft degrees of Freemasonry, conferred after Entered Apprentice and Fellow Craft. Holding it is the standard entry requirement for the appendant bodies that confer further degrees, including the York Rite's Chapter, Council and Commandery, and the Ancient and Accepted Scottish Rite's Lodge of Perfection, Chapter of Rose Croix, Council of Kadosh and Consistory. In that sense Master Mason functions as a gateway degree for the whole further landscape of organized Freemasonry rather than only as the top of the basic Lodge sequence.
Freemasonry's own literature is emphatic that this structural role does not make anything beyond Master Mason a higher rank in Masonic standing. Everything conferred by the York Rite, the Scottish Rite or any other appendant body is described as an addition to Craft membership, not a promotion past it; a thirty third degree Scottish Rite Mason remains, in Craft terms, a Master Mason who has also received further degrees elsewhere.
The historical scholarship on early organized Freemasonry, tracing from the 1717 founding of the Grand Lodge of England, situates the settled three degree Craft system, with Master Mason as its capstone, as a development of the first half of the eighteenth century, arrived at gradually rather than fixed from the outset, alongside the broader eighteenth century politics that shaped how organized Freemasonry took its modern form.