"And she shall remove his shoe off his foot"Deuteronomy 25:9.

If the brother of a married man who dies childless does not marry his widowed sister-in-law [See Positive Commandment 216], the widow is commanded to remove her brother-in-law's shoe [as part of the chalitzah ceremony described in the Torah, and she is then permitted to marry whomever she desires].

Our sages tell us that performing levirate marriage is preferable to employing chalitzah to release the widow and brother-in-law from their bond.