The Summer Triangle is formed by three of the brightest stars in the summer sky. On the west corner of the triangle is Vega, the 5th brightest star in the night sky. On south corner is the first magnitude star Altair in the constellation Aquila. At the northeast corner is the star Deneb in Cygnus.
The Summer Triangle is almost 40 degrees in diameter, and it intersects the constellations Lyra, Cygnus and Aquila. Look for this large asterism in the evening sky during the summer and early fall.
For southern hemisphere observers, these stars are best seen during winter months. But as this asterism lies relatively far to the north, it never appears far from the horizon. This is why the "Summer Triangle" nickname used by Northerners is much more common.