When you really like the best of the best regarding your suits and dress style then fittings need to happen. The most traditional process of having a bespoke suit tailored involves several fittings.
The main fitting is the so-called Baste Fit. Also referred to as the Toile Fitting. This is when your suit/ jacket is loosely put together with white threads. See this as a prototype and early warning system. This so that the tailor knows even better what your personal preferences are regarding cut, fit, drape, and personal details.
After the fitting the tailor will proceed with adapting the garment to your wishes as discussed together so it is ready for the next fit.
Hopefully this gives you a better impression of how a tailor and a baste fit works.
Kind regards, Jean-Paul Samson