The Summer Triangle (a.k.a. Summer-Autumn Triangle) is an imaginary triangle on the celestial sphere consisting of Vega in Lyra, Altair in Eagle, and Tientsin in Cygnus, with Vega at the right-angled apex of the triangle. These three stars are the brightest stars in their respective constellations. During the summer months, the Summer Triangle rises to the zenith in the mid-latitudes of the Northern Hemisphere. The Summer Triangle can still be easily seen in the west after sunset in the fall.

Reference: zh. *** /wiki/%E5%A4%8F%E5%AD%A3%E5%A4%A7%E4%B8%89%E8%A7%92