triangular recess

tri·an·gu·lar re·cess

an occasional evagination of the anterior wall of the third ventricle of the brain between the anterior commissure and the diverging pillars of the fornix.
References in periodicals archive ?
It was designed by Charles Cockerell and carved by William Nichollforthe large outside triangular recess at the top on the south sideof the hall.
Slide the triangular adapter piece onto the shaft and align with the triangular recess in the trimmer head.