At an intersection unregulated by a traffic light, making a left turn by pulling halfway out into the road to block oncoming traffic while waiting for a gap in the other lane.

So common in Massachusetts that drivers will stop and wait for a left-turner, even when they have the right of way.

While stopped at an intersection, A Massachusetts Left is the act of quickly taking a left as soon as the light turns green, in order to avoid having to wait for the line of cars heading your way to pass.