In
the Arthurian mythos, Kai ( alternatively Kay or Cai ) was Arthur's foster brother, the natural son of Ector.
Thus Kai was one of Arthur's most loyal supporters. In later years Kai ran
Arthur's household as his steward or seneschal collecting his revenues

John
Morris his book 'The Age of Arthur' has this to say on the collection of
revenues in this period;-
'But the British kings did not acquire their revenues
in the manner of the Irish.
The Roman past offered a better model. Like the Irish Kings, Maelgwn and other
British rulers levied a tribute of cattle, corn and other dues. In Ireland each
lesser king was required to deliver dues to his superior, and to receive gifts
in return. Payments depended on continuing loyalty, and loyalty was enforced by
obliging the lesser king to deposit one or more of his sons as hostages at the
court of his superior. The British kings found more efficient means of levying
tribute; the stories of the Welsh monks are stuffed with dramatic miracles whose
occasion is the visit of royal officers demanding tribute and maintainance for
their men.
The laws of ancient Wales detail the functioning of such officers, and some of
them are recognisably Roman. Late Roman military commanders were entitled to
receive fixed quantities of annona, supplies and maintainance, whose collection
was supervised by erogatores, armed with warrants authorising the requisition of
stated commodities from named villages. In fifth-century Gaul the Goths were
billeted on landowners by Roman law, and received annona also collected by
erogatores. In Gaul and Italy, the title and function of the erogator militaris
annona long outlasted the days of the emperors; and in fifth-century
Britain Gildas described the supplies due to the English federates by the
technical term annona. There is little doubt that in Britain too they were
collected through erogatores.
In medieval welsh law the revenue officers were termed 'Cais'. The word is formed
from ceiso, to ask, seek, search for, and is the linguistic equivalent of the
Latin erogator, 'a person who asks' for revenue on behalf of the state. Like the
erogator, the cais visited each district in turn to collect its stated tribute,
in a circuit called cylch. The tributes were those which the Saint's Lives
condemn, a 'cornage' of miscellaneous foodstuffs, with a cow payment, and,
dofraeth, billeting and maintainance. But the Welsh system is not confined to
Wales; it persisted until the twelfth-century throughout Northern England from
Cheshire to Northumberland and Durham, and in the Clyde kingdom, and was
accompanied by much Welsh land tenure. The term cais was commonly Englishised as
Keys, or was translated by 'serjeant'; and the serjeant made similar circuits,
collecting cornage, cowgeld and maintainance.
The elements of the system are simple and, might naturally arise in a simple
society. But they differ radically from the practice of the rest of England; and
when identical customs, and identical terms are found in Wales, northern England
and southern Scotland, then it is evident that their origin dates back to a time
before these territories were separated from each other by English conquests in
the late sixth-century.' -
The Age of Arthur - John Morris
The correlation between 'Cai', Arthur's
seneschal, and 'Cais' the medieval welsh term for a seneschal almost certainly
means that at some point during the written, or oral, transmission of the mythos
someone confused name and jobtitle
and the name has stuck ever since. This confusion is common, if hard to
disentangle, in the transmission of early tales
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