A Cristophere on his brest of silver sheene.
An horn he bar, the bawdryk was of grene.
A forster was he, soothly as I gesse.

from Geoffrey Chaucer – The Canterbury Tales (General Prologue) on Rap Genius

Meaning

(= a bright silver medallion of St. Christopher on his chest / and a horn he carried. The baldric [shoulder strap] was coloured green. He was a forester, it seemed to me.)

St. Christopher was the patron saint of travellers. As painted by Titian:

The Yeoman is going travelling, so it makes sense to wear such a medallion.

The overall effect of the Yeoman’s description is essentially to enhance the Knight’s humble and unpretentious portrayal. The Yeoman himself doesn’t have much personality, besides being extremely good at his occupation.

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