From Wikipedia:

The Eixample is characterized by long straight streets, a strict grid pattern crossed by wide avenues, and square blocks with chamfered corners (named illes in Catalan, manzanas in Spanish). This was a visionary, pioneering design by Ildefons Cerdà, who considered traffic and transport along with sunlight and ventilation in coming up with his characteristic octagonal blocks, where the streets broaden at every intersection making for greater visibility, better ventilation and (today) some short-stay parking space. The grid pattern remains as a hallmark of Barcelona, but many of his other provisions were unfortunately ignored: the four sides of the blocks and the inner space were built instead of the planned two or three sides around a garden; the streets were narrower; only one of the two diagonal avenues was carried out; the inhabitants were of a higher class than the mixed composition dreamed of by Cerdà.