书中第二章讲创建一个场景节点时，说左右手坐标系：
The names left-and right-handed are based on the fact that the orientation of the axis can be reconstructed using the left and right hand. The thumb is the x-axis, the index finger the y-axis, and the middle finger the z-axis. We need to hold our hands so that we have a ninety-degree angle between thumb and index finger and also between middle and index finger. When using the right hand, we get a right-handed coordination system. When using the left hand, we get the left-handed version.
【 左手和右手坐标系名字的由来是基于他们主轴方向的不同，方向是左手和右手来决定创建的。大拇指是X轴，食指是Y轴，而中指是Z轴。我们需要张开手使大拇指，食指和中指之间保持90度角。当使用右手时，我们就可以得到一个右手坐标系。当使用左手时，我们就可以得到一个左手坐标系】
说的很妙，我也就知道了为什么是分左右手的了。保持姿势左手没法逆时针，保持姿势右手没法顺时针。