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human
[The Princess Bride] Why does Westley choose to use his left hand when dueling Inigo?

He is pursuing his true love in an attempt to rescue her from kidnappers who plan to kill her. It seems time would be of the essence, so why does he choose to dual with his non-dominant hand?
gpt
He was giving his right arm a break. He'd just climbed up a cliff. Inigo gave him a rest but Westley probably felt he could use more time.  Westley was also confident in his left-handed swordfighting.

As a master swordsman himself, it's also possible that Westley could tell that Inigo was right-handed. He guessed at Inigo's trick, and so kept his own right hand in reserve.

EDIT: It's also possible that Westley knew about trio, at least in the sense that each of them existed. Who builds up an immunity to iocane powder for no reason?