What are the things that friendship is compared to in the poem of sonnet 87 by Shakespeare? 

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sciftw eNotes educator| Certified Educator

Most of the comparisons being made between the narrator and his supposed wealthy benefactor or lord are comparisons using legal metaphors.  The most noticeable legal metaphor of their friendship is when the narrator of the poem refers to it as a "charter."  In the world of business, a charter would give a corporation certain privileges.  It's likely that the poet's friendship gave the narrator the same kind of privileges.  If indeed the friend is a wealthy lord, then those privileges may have been access to high class society or even financial support.  

Another legal comparison is found in the word "patent."  The poet says that he got his patent back again, which means the lord is no longer holding it. It's clear that the two were friends, but it's also clear through the friendship comparisons that the friendship was dependent on how the poet could benefit the lord.  Perhaps the lord found another "patent" with which to make his charter.  Whatever the case may be, the poem makes it clear that the friendship is over.