Affine open cover
= Affine open cover
{title2=$X=\bigcup_iU_i,\quad U_i\cong\operatorname{Spec}A_i$}
An affine open cover of a <scheme> is an <open cover> by <affine open subsets>. Every <scheme> has such a cover by definition. A <quasi-compact scheme> has a finite affine open cover. Whether the intersections are affine is a further condition, supplied in particular by separatedness over an affine base.