diff --git a/SketchWidget.py b/SketchWidget.py
index 418a3df..76185f4 100644
--- a/SketchWidget.py
+++ b/SketchWidget.py
@@ -106,14 +106,14 @@ class SketchWidget:
         glBegin(GL_QUADS)
         angle = math.atan2(y1 - y0, x1 - x0)
         ninety = math.pi / 2
-        glVertex(x0 + size * math.cos(angle + ninety),
-                y0 + size * math.sin(angle + ninety))
-        glVertex(x0 + size * math.cos(angle - ninety),
-                y0 + size * math.sin(angle - ninety))
-        glVertex(x1 + size * math.cos(angle - ninety),
-                y1 + size * math.sin(angle - ninety))
-        glVertex(x1 + size * math.cos(angle + ninety),
-                y1 + size * math.sin(angle + ninety))
+        x_left = size * math.cos(angle + ninety)
+        y_left = size * math.sin(angle + ninety)
+        x_right = size * math.cos(angle - ninety)
+        y_right = size * math.sin(angle - ninety)
+        glVertex(x0 + x_left, y0 + y_left)
+        glVertex(x0 + x_right, y0 + y_right)
+        glVertex(x1 + x_right, y1 + y_right)
+        glVertex(x1 + x_left, y1 + y_left)
         glEnd()
 
     def drawFilledCircle(self, x, y, radius, steps):