A method of forming a fluid-filled chamber (300, 300a, 300b) for incorporating into an article of footwear, the method comprising: positioning a first polymer sheet (301, 301a, 301b) on a substantially first flat surface of a tool (500, 500a); positioning a second polymer sheet (302, 302a, 302b) on the first polymer sheet (301, 301a, 301b); moving a second substantially flat surface of the tool (500, 500a) into contact with the second polymer sheet (302, 302a, 302b); maintaining a gap between the first polymer sheet (301, 301a, 301b) and the second polymer sheet (302, 302a, 302b) at a predetermined area (340, 340a, 340b); heating one of the two substantially flat surfaces of the tool (500, 500a) to heat one of the first polymer sheet (301, 301a, 301b) and the second polymer sheet (302, 302a, 302b); joining the first polymer sheet (301, 301a, 301b) and the second polymer sheet (302, 302a, 302b) together at locations outside of the predetermined area (340, 340a, 340b) to define a peripheral bond (350, 350a) of the chamber (300, 300a, 300b); maintaining the gap between the first polymer sheet and the second polymer sheet includes adding a tensile element defining the predetermined area between the first polymer sheet and the second polymer sheet; adding the tensile element defining the predetermined area between the first polymer sheet and the second polymer sheet includes positioning a first tensile layer of the tensile element in contact with the first polymer sheet and positioning a second tensile layer of the tensile element in contact with the second polymer sheet; heating one of the two substantially flat surfaces of the tool bonds one of the first polymer sheet and the second polymer sheet to a respective one of the first tensile layer and the second tensile layer; moving the second substantially flat surface of the tool away from the second polymer sheet after joining the first polymer sheet and the second polymer sheet; positioning the second polymer sheet on the substantially first flat surface of the tool; moving the second substantially flat surface of the tool into contact with the first polymer sheet; and heating the one of the two substantially flat surfaces (501, 502) of the tool to heat the other of the first polymer sheet and the second polymer sheet. positioning a first polymer sheet (301, 301a, 301b) on a substantially first flat surface of a tool (500, 500a); positioning a second polymer sheet (302, 302a, 302b) on the first polymer sheet (301, 301a, 301b); moving a second substantially flat surface of the tool (500, 500a) into contact with the second polymer sheet (302, 302a, 302b); maintaining a gap between the first polymer sheet (301, 301a, 301b) and the second polymer sheet (302, 302a, 302b) at a predetermined area (340, 340a, 340b); heating one of the two substantially flat surfaces of the tool (500, 500a) to heat one of the first polymer sheet (301, 301a, 301b) and the second polymer sheet (302, 302a, 302b); joining the first polymer sheet (301, 301a, 301b) and the second polymer sheet (302, 302a, 302b) together at locations outside of the predetermined area (340, 340a, 340b) to define a peripheral bond (350, 350a) of the chamber (300, 300a, 300b); maintaining the gap between the first polymer sheet and the second polymer sheet includes adding a tensile element defining the predetermined area between the first polymer sheet and the second polymer sheet; adding the tensile element defining the predetermined area between the first polymer sheet and the second polymer sheet includes positioning a first tensile layer of the tensile element in contact with the first polymer sheet and positioning a second tensile layer of the tensile element in contact with the second polymer sheet; heating one of the two substantially flat surfaces of the tool bonds one of the first polymer sheet and the second polymer sheet to a respective one of the first tensile layer and the second tensile layer; moving the second substantially flat surface of the tool away from the second polymer sheet after joining the first polymer sheet and the second polymer sheet; positioning the second polymer sheet on the substantially first flat surface of the tool; moving the second substantially flat surface of the tool into contact with the first polymer sheet; and heating the one of the two substantially flat surfaces (501, 502) of the tool to heat the other of the first polymer sheet and the second polymer sheet.