The very large size of megacrysts in granitoids reflects the relative difficulty of nucleation of K-feldspar in silicate liquids. At least 92% of a large population of K-feldspar megacrysts separated by weathering from a granitoid show simple twinning, supporting observations of typical megacrystic granitoids and trachytes that suggest simple twinning is very common in igneous K-feldspar. This abundance implies that simple twinning strongly promotes the development of viable crystals of K- feldspar in silicate liquids. The similar size of each twin sub-individual indicates twin formation at or soon after nucleation. K-feldspar grown interstitially shows simple twinning much less commonly than K-feldspar grown freely in liquid, probably because abundant crystals are available to act as nucleation substrates. The rarity or absence of simple twinning in the K-feldspar of metamorphic rocks presumably also reflects abundant existing crystals acting as heterogeneous nucleation substrates.