= K3 period domain
{c}
{title2=$\{[\omega]:q(\omega,\omega)=0,\ q(\omega,\bar\omega)>0\}$}
On the <K3 intersection lattice>, this open quadric has complex dimension twenty and records the period line $H^{2,0}$. Fixing a positive class $h$ of type $(1,1)$ adds $q(h,\omega)=0$ and yields a nineteen-dimensional domain for primitive polarized cohomology of signature $(2,19)$. The distinction is necessary: the unrestricted full lattice has signature $(3,19)$ and does not satisfy polarization positivity on all of $H^{1,1}$.
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