Duplicate stitch traces the exiting stitches knit in stockinette. Often used as a decorative accent, it can also be used to mask an error. The trick is sliding the needle behind both legs of the stitch above the one you’re tracing, which can feel counter-intuitive; see image below.
A demonstration of how to do duplicate stitch is available in the following video. If you want to knit letters, my go-to resource is from Purl Soho.
Want to Give it a Try?
The following pattern uses duplicate stitch:
Cushion of Joy 