Vintage T-Shirt Forum & Community Authentication Appraisals

If I see a single line of thread on the outside of a shirt (on the bottom hem or shirt sleeve), but the inside stitching looks like a double stitch - what type of stitching is this, single or double?

I am going with the assumption that seeing a double row of threads on the outside of the shirt is double-stitch. I can't seem to come to the same conclusion in my question.

Also, is the collar stitching taken into account?

Thank you!

So yeah good Q, it's what's visible from the outside. It's usually based on the arm and lower hem. Collar/shoulder can often be double while the hems are single, and it's considered single.

Sometimes there's dual stitching!

https://www.defunkd.com/single-stitch/

Thanks for the clarifying info!