Good edge of a dual line arrangement
= Good edge of a dual line arrangement
= Good dual-arrangement edge
{synonym}
A <good dual-arrangement edge> has degree-six endpoints and two triangular adjacent faces. Other edges are bad. The <Euler defect identity for a projective line arrangement> bounds the number of bad edges by $16t_2$: charge them to degree-four or higher-degree vertices and to nontriangular faces. This turns a count of <ordinary lines> into quantitative control of local triangular geometry.