Singer Futura XL-550 Manuel D'instruction page 58

9. When sewing is finished, use a buttonhole opener to open
up the fabric in the center of the buttonhole.
Be careful not to cut any of the bartack threads.
CORDED BUTTONHOLES
Hook filler cord (crochet thread or buttonhole twist) over the
spur and pull both ends of the cord forward under the foot and
tie off the cord as shown in the illustration.
Sew buttonhole so that zigzag stitches cover the cord.
When completed, release the cord from foot and snip off extra
length.
BUTTONHOLING WITH HARD-TO-SEW
FABRICS
When buttonholing with hard-to-sew fabrics or along the edge
of multi-layered garments, attach the under-plate (A) onto the
buttonhole foot as illustrated to make perfect buttonholes.
Place the fabric between the under-plate and buttonhole foot.
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Spur
Saliente
Tige
(A)