The upper thread (needle thread) runs from the thread spool via the arm of the sewing machine through the thread guide with pre-tensioning device, thread tensioning device, thread take-up lever, thread take-up spring, and thread guide with guide eye to the sewing needle. With the upper thread tension set in advance, it forms the upper part of the seam and is fed via the sewing needle during sewing and connected to the lower thread (spool thread) during interlacing.
Source:
Eberle, Hannelore; Gonser, Elke; Hornberger, Marianne; et al. (2022), Fachwissen Bekleidung, 12. Auflage, Haan-Gruiten: Verlag Europa-Lehrmittel, S. 211