"The direction of the main axis of the el ipse with respect to the plumb line defines the directionality angle, or gamma of a stain. That can range from zero to three-sixty.

"The impact angle, or alpha, can range from zero to ninety degrees. That's calculated from the shape of the el ipse."

"Why is that?"

"Remember, when a drop of blood travels through space it's spherical. But when it strikes a target it flattens and leaves a trail. That's because the bottom of the drop is actual y wiping across the surface."

He made a swiping gesture with his hand.

"The trail is smal at first as the drop strikes, then widens, with the widest point of the trail corresponding to the center, or widest part of the drop. The trail then narrows and eventual y tapers off. See this one here?"

He pointed to an elongated oval with a smal dot at one end. It looked like many I'd seen in the blood-spatter room.

"It looks like an exclamation point."

"That's exactly what it's cal ed. Sometimes a smal dot of blood detaches from the original drop and hops to the head of the trail. So looking at a spatter from above, it resembles either a tadpole or an exclamation point, depending on whether the far end is merely elongated, or a smal portion detached completely. In either case the direction of travel is clear."

"The dot points toward the direction the drop was moving."

"Exactly. The program produces a file containing the values of the angles for each stain analyzed. It's from that data that point of origin is calculated.

And believe me, using the computer is a lot quicker than the old string method."

"Back up."

"Sorry. With the string method one end of a string is fastened to the surface at the position of the stain, then stretched in the estimated direction of motion. This is repeated for a number of bloodstains around the scene. The result is a pattern of strings extending away from the spatter toward the source of the blood. Home plate is the point where al the strings converge. The procedure is time consuming and leaves a lot of room for error. Instead of doing it by hand the computer draws virtual strings computed from the data."

His fingers flew over the keys and a new image appeared. X and Y coordinates ran down the left side and across the bottom of the screen. A dozen lines formed an X-shaped pattern, crossing each other in a geometric bow.

"This is a bird's-eye view of a set of virtual strings based on twelve spatters. It's hard to get this point of view with real strings, yet it's the most useful one."