This reed spline is used to fasten the machine woven cane into the grooves on the perimeter of the chair seat.  It is sold in 3 widths as follows:

If the width of the groove in your chair is 11/64" you should order the Small spline.
If the width of the groove in your chair is 3/16" you should order the Medium spline
If the width of the groove in your chair is 7/32" you should order the Large spline.

Sold by the running foot..