diff --git a/gfx/2d/PathCG.cpp b/gfx/2d/PathCG.cpp
index de1205b7cd3..322731407a6 100644
--- a/gfx/2d/PathCG.cpp
+++ b/gfx/2d/PathCG.cpp
@@ -68,12 +68,20 @@ void
 PathBuilderCG::Arc(const Point &aOrigin, Float aRadius, Float aStartAngle,
                  Float aEndAngle, bool aAntiClockwise)
 {
+  // Core Graphic's initial coordinate system is y-axis up, whereas Moz2D's is
+  // y-axis down. Core Graphics therefore considers "clockwise" to mean "sweep
+  // in the direction of decreasing angle" whereas Moz2D considers it to mean
+  // "sweep in the direction of increasing angle". In other words if this
+  // Moz2D method is instructed to sweep anti-clockwise we need to tell
+  // CGPathAddArc to sweep clockwise, and vice versa. Hence why we pass the
+  // value of aAntiClockwise directly to CGPathAddArc's "clockwise" bool
+  // parameter.
   CGPathAddArc(mCGPath, nullptr,
                aOrigin.x, aOrigin.y,
                aRadius,
                aStartAngle,
                aEndAngle,
-               !aAntiClockwise);
+               aAntiClockwise);
 }
 
 Point