Help! What is this piece of material that holds the pieces together?! Not the rivet but the lil fabric in between the rivet and the actual designed material. (Thanks for the help! :) )

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Help! What is this piece of material that holds the pieces together?! Not the rivet but the lil fabric in between the rivet and the actual designed material. (Thanks for the help! :) )

Solved )-A single riveted lap joint is made in 15 mm thick