MACROMEDIA FLASH 8-LEARNING ACTIONSCRIPT 2.0 IN FLASH Manual page 553

You can set the following three types of joint styles for the
(default)
round
miter
bevel
The following example demonstrates the differences between each of the three joint styles.
To set different joint styles:
1.
Create a new Flash document and save it as jointstyles.fla.
2. Add the following ActionScript to Frame 1 of the Timeline:
var lineLength:Number = 100;
// miter
this.createEmptyMovieClip("miter_mc", 10);
miter_mc.lineStyle(25, 0xFF0000, 100, true, "none", "none", "miter",
25);
miter_mc.moveTo(0, lineLength);
miter_mc.lineTo(lineLength / 2, 0);
miter_mc.lineTo(lineLength, lineLength);
miter_mc.lineTo(0, lineLength);
miter_mc._x = 50;
miter_mc._y = 50;
var lbl:TextField = miter_mc.createTextField("label_txt", 10, 0,
lineLength + 20, lineLength, 20);
lbl.autoSize = "center";
lbl.text = "miter";
// round
this.createEmptyMovieClip("round_mc", 20);
round_mc.lineStyle(25, 0xFF0000, 100, true, "none", "none", "round");
round_mc.moveTo(0, lineLength);
round_mc.lineTo(lineLength / 2, 0);
round_mc.lineTo(lineLength, lineLength);
round_mc.lineTo(0, lineLength);
round_mc._x = 200;
round_mc._y = 50;
var lbl:TextField = round_mc.createTextField("label_txt", 10, 0,
lineLength + 20, lineLength, 20);
lbl.autoSize = "center";
lbl.text = "round";
// bevel
this.createEmptyMovieClip("bevel_mc", 30);
bevel_mc.lineStyle(25, 0xFF0000, 100, true, "none", "none", "bevel");
bevel_mc.moveTo(0, lineLength);
bevel_mc.lineTo(lineLength / 2, 0);
bevel_mc.lineTo(lineLength, lineLength);
bevel_mc.lineTo(0, lineLength);
bevel_mc._x = 350;
bevel_mc._y = 50;
parameter:
jointStyle
Drawing with ActionScript 553