<title>Abstract</title>Atom probe tomography (APT) and transmission electron microscopy (TEM)/scanning transmission electron microscopy (STEM) have been used correlatively to explore atomic-scale local structure and chemistry of the exactly same area in the vicinity of growth front of a long-period stacking ordered (LPSO) phase in a ternary Mg–Al–Gd alloy. It is proved for the first time that enrichment of Gd atoms in four consecutive (0001) atomic layers precedes enrichment of Al atoms so that the formation of Al₆Gd₈ clusters occurs only after sufficient Al atoms to form Al₆Gd₈ clusters diffuse into the relevant portions. Lateral growth of the LPSO phase is found to occur by ‘ledge’ mechanism with the growth habit plane either {1<inline-formula><alternatives><tex-math>$$\overline{1}$$</tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
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</mml:math></alternatives></inline-formula>00} or {11<inline-formula><alternatives><tex-math>$$\overline{2}$$</tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
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</mml:math></alternatives></inline-formula>0} planes. The motion of ledges that give rise to lateral growth of the LPSO phase is considered to be controlled by diffusion of Al atoms.