Dear Jan,
How important are the spool caps, and how do I determine which size to use? -Anonymous

Dear Anonymous,
Spool cap covers prevent thread from catching in the spool notch, and assure even delivery of thread. Ordinary spools require the large spool cap, which is larger in diameter than the thread spool, and thinner spools use the small cap cover, which is larger in diameter than the spool, and yet not too large as to impede thread delivery. When using metallic threads, it may be more desirable to stand the spool on the auxiliary spool pin or on an accessory thread stand, with the large spool cap underneath the spool, with the flat side to spool end.