Because Tereus had cut out Philomela's tongue (so she couldn't tell anyone he had raped her), she wove a tapestry showing the events and sent it to her sister, Procne.
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Philomela wove a tapestry that depicted the truth of Tereus's evil deeds and sent it to Procne as a secret message. This allowed Philomela to communicate with her sister without Tereus knowing.