for hand applique i use silk thread, it comes in so many wonderful colors, is thin, strong and disappears - it is very smooth -glides nicely. for machine applique i use what every thread is the correct color and weight for what i am appliquing-- for wool applique i use wool threads, and purol cottons most of the time but also use silk floss and other specialty flosses/threads. 50-weight or higher (the higher the number the thinner the thread) in a color that matches or is one shade darker- or a neutral the will blend.