This article from Threads #177 (February/March 2015) is by Threads contributing editor Claire Shaeffer. She writes:

Button loops, thread eyes, bar tacks, French tacks, lingerie guards, and belt carriers have something in common. They’re frequently made from thread bars or chains and are intended to provide a soft, subtle way to secure garment elements without showing. These delicacies are created several ways – the bars are made by handsewing buttonhole or blanket stitches over foundation thread strands. Make thread chains by crocheting with your fingertips. Thread chains are softer than thread bars, but they are less durable. I’ll explain how to make both and where to use them.