Quilting Dictionary Half-Square Triangle (HST)PiecingShare this Facebook Pinterest X WhatsApp Email Copy linkA pieced unit made from two right triangles sewn together along their diagonal to form a square — one of the most common building blocks in quilting. Half-Square Triangle (HST) Quarter-Square Triangle (QST) Half-Rectangle Triangle (HRT) Half-Square Triangle (HST) Geometry Two right-angle triangles joined along their hypotenuse diagonal to form a clean square unit. 1. Diagonal Seam Lines2. Pressed Square UnitStraight Grain Also see QuiltingBlock