This entry titled Golden Triangle actually refers to an area of Milan in the shape of a rectangle - quadrilatero is the Italian word for quadrilateral or rectangle. This particular rectangle is bordered by Via Montenapoleone, Via Borgospesso, Via Della Spiga, and Via Sant' Andrea.

This area is where all the major fashion designers have their stores.  If you are not interested in top-class shopping you might as well skip this area entirely, although on Via Sant'Andrea there are two interesting (and pretty much unknown) museums: http://www.museodimilano.mi.it/ and http://www.museodistoriacontemporanea...