What is claimed is:1. A retainer for filamentary material wound on a cylindrical surface of a drum between spaced parallel flanges of a bobbin, said retainer comprising:a) a strip of elastic material having first and second ends and a predetermined width;b) said first end being folded back upon and superposed with an intermediate portion of said strip to form a minor loop extending from said first end to said intermediate portion;c) said second end being superposed with one of said first end and said intermediate portion to form a major loop, significantly larger than said minor loop with three layers of said material mutually superposed in the areas of said intermediate portion and adjacent said first and second ends;d) said major loop having a circumference in the unstretched condition somewhat smaller than that of said bobbin drum;e) means permanently securing said three layers in mutually engaged relation; andf) means defining an opening extending through all of said three layers.