Small particles with face-centred cubic structures can have non-single-crystallographic shapes. Here, an approach based on annular dark-field scanning transmission electron microscopy (STEM) is used to obtain information about the crystal sub-units that make up supported and unsupported twinned Pt, Pt alloy and Au nanoparticles. The three-dimensional shapes of two types of lamellar-twinned particles (LTPs) of Pt are obtained using high-angle annular dark-field STEM. Possible growth mechanisms of the LTPs and origins for the contrast features in the recorded images are discussed.