thread size

[The Wizard of BC]

Gutermann sew-all is what I use and what is recommended in The Art of Kiltmaking. Insure you get the 100% polyester.
This is for sewing the Fell, buckle tabs and any part that would show. The goal is no visible stitches. None, nada.
Use this same thread for the interior details such as the facings

If you can, use the exact color of the place you are sewing. Blue for the Blue pleats, Green for the Green pleats etc. Change color as needed.
If you need to choose between two colors take the one slightly darker.

For sewing the stabilizer, interfacing and straps I use upholstery thread. This is not according to TAoK but I want strength over invisible stitches in these places.

For basting I use any thread I have at hand. I like upholstery thread for this, but that is because I have fat, guy fingers and I can take out the basting, iron the thread and use it again. Barb thinks I'm insane.