Monofilament thread is used in places where you want the thread to be invisible or as inconspicuous as possible.
How to Choose Thread: What You Need to Know - WeAllSew
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How to use MonoPoly Thread in Your Sewing Machine
fussy cutting Night Quilter
A Guide to Using Monofilament Thread - WeAllSew
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Tips For Working With Invisible Monofilament Thread Quilting Digest
Tips for Working with Invisible Monofilament Thread - Quilting