The polydomain-monodomain (PM) transformation takes place when a polydomain of a smectic-C main-chain liquid-crystalline elastomer (SmC MCLCE) is uniaxially stretched. We present results based on a combination of mechanical and X-ray experiments which show how the domains initially rearrange to finally form a perfect conical layer distribution (monodomain) when the sample is fully stretched. The rearrangement and orientational process of the domains is quantified and compared to the parallel and perpendicular uniaxial stress-strain deformations of a monodomain sample. The stress-strain behaviour of the polydomain lays between the uniaxial deformations, parallel and perpendicular to the director, of the monodomain sample.