be memorized. For the top sign, the arrows show that the two '1' hands move in vertical circles, and that although they move at the same time (tie bar), the left hand (hollow arrowhead) starts away from the body (thin line) going up while the right hand (solid arrowhead) starts near the body (thick line) going down. With the bottom sign, the right 'X' palm-down hand moves down-side-down relative to the stationary palm-up 'B' hand. This is overly exact: The ASL sign will work with any downward zigzag motion, and the direction and starting point of the circles is irrelevant. Finger movement There are only a few symbols for finger movement. They may be doubled to show that the movement is repeated. A solid bullet represents flexing the middle joint of a finger or fingers, and a hollow bullet represents straightening a flexed finger. That is, a 'D' hand with a solid bullet means that it becomes an 'X' hand, while an 'X' hand with a hollow bullet means that it becomes a 'D' hand. If the fingers are already flexed, then a solid bullet shows that they squeeze. For example, a square (closed fist, 'S' hand) with double solid bullets is the sign for 'milk' (iconically squeezing an udder). A downward-pointing chevron represents flexing at the knuckles, while an upward-pointing chevron (^) shows that the knuckles straighten. That is, a 'U' hand with a down chevron becomes an 'N' hand, while and 'N' hand with an up chevron becomes a 'U' hand. A zigzag like two chevrons (^^) joined means that the fingers flex repeatedly and in sync. A double-line zigzag means that the fingers wriggle or flutter out of sync. Hand movement Hundreds of arrows of various sorts are used to indicate movement of the hands through space. Movement notation gets quite complex, and because it is more exact than it needs to be for any one sign language, different people may choose to write the same sign in different ways. For movement with the left hand, the Δ-shaped arrowhead is hollow (white); for movement with the right hand, it is solid (black). When both hands move as one, an open (Λ-shaped) arrowhead is used. As with orientation, movement arrows distinguish two planes: Movement in the vertical plane (up & down) is represented by arrows with double stems, as at the bottom of the diagram at left, while