None.

[C#]

using (Bitmap bm = new Bitmap(600, 600)) {
  using (Graphics graphics = Graphics.FromImage(bm)) {
    GraphicsPath big = new GraphicsPath();
    big.AddEllipse(-2, -2, 4, 4);
    GraphicsPath small = new GraphicsPath();
    small.AddEllipse(-1, -1, 2, 2);
    Pen pen = new Pen(Color.Black, 20);
    pen.CustomStartCap = new CustomLineCap(null, big);
    pen.CustomEndCap = new CustomLineCap(small, null);
    for (int i = 0; i < 3; i++) {
      pen.CustomStartCap.BaseInset = i;
      pen.CustomEndCap.BaseInset = i;
      pen.Color = Color.Red;
      graphics.DrawLine(pen, 100, 100, 200, 200);
      pen.Color = Color.Green;
      graphics.DrawLine(pen, 200, 200, 100, 400);
      pen.Color = Color.Blue;
      graphics.DrawLine(pen, 100, 400, 200, 500);
      graphics.TranslateTransform(150, 0);
    }
  }
  bm.Save(Server.MapPath("IG8_CustomLineCap.png"));
}

The code here uses two different custom line caps. We vary the BaseInset to show how it affects the end point of the line. The final output is shown below. If you are interested in designs for custom end points HTML Arrows is a good place to start.

IG8_CustomLineCap.png