What stitch type, characterized by using a double thread, was characteristic of reliable, functional sewing machines?

Answer

The lockstitch.

The key to the successful, large-scale operation of the sewing machine was the type of stitch it produced. The reliable, functional machines developed in the mid-1800s utilized a double thread to create the lockstitch. This particular stitch mechanism resulted in a seam that was inherently stronger and significantly faster to create than the traditional chainstitch or running stitch that was executed manually by hand sewers.

What stitch type, characterized by using a double thread, was characteristic of reliable, functional sewing machines?
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