Final answer:
A shift in the central ideas of a sonnet is called a volta. In Shakespeare's 'Sonnet 91', the volta shifts the theme from material wealth to the emotional value of love and affection.
Step-by-step explanation:
The shift in the central ideas of a sonnet is commonly referred to as a volta. A volta typically occurs between the octet (first eight lines) and the sestet (last six lines) in Petrarchan sonnets, or it can appear between lines eight and nine or between lines twelve and thirteen in Shakespearean sonnets.
The central idea of Shakespeare's 'Sonnet 91' undergoes a notable shift. Initially, the speaker discusses various forms of wealth and riches but concludes that his greatest treasure is the love and affection he shares with his beloved, which he values far above material wealth.
This turn in thought signifies the poem's volta, effectively shifting from a material to an emotional focus.