Twinning is the symmetric intergrowth of two or more crystals of the same mineral, and commonly is revealed in thin section by the different crystals of the same mineral having different extinction positions or different interference colours.  In A the polysynthetic twinning (Albite Law) of plagioclase is evident in the “zebra” effect of adjacent twins having slightly different shades of first order grey to white colours.  Two grains at the bottom centre of the image also show simple Carlsbad twinning.  In B, deformation induced twinning is apparent in calcite, evident as pastel colour bands in grains that are otherwise extinct or nearly extinct.  In calcite, these twin lamellae are parallel to the rhombic cleavage or crystal outlines.  Simple twins are illustrated in C and D.  In C, Carlsbad twins on {010} are evident in several phenocrysts and microphenocrysts of sanidine in a trachyte and in D, a phenocryst of hornblende in an andesite is twinned on {100}.   Views A - C are 5.5 mm across and D is 2.2 mm across, and all are in x-nicols.