This year marks the 80th anniversary of the publication of the CIE 1931 Standard Observer for color measurement and specification. This is the basic standard that makes possible the plots you see of primary color triangles on the background of the "horseshoe" shaped spectral locus. It lead to the scientific design of color reproduction systems, derivative plots that indicate color gamut and reproduction accuracy, matrix calculations to improve the color rendition of color TV cameras and digital still cameras, and so on.

http://en.wikipedia.org/wiki/CIE_1931_color_space

http://www.videokarma.org/attachment.php?attachmentid=166613&d=1269294026
I thought it looked more like a shark's fin than a horseshoe... :)

It is also sometimes called a tongue shape.

Another good explanation of the derivation of the CIE diagram is in

Fink, Donald G., Television Engineering 2nd. Ed. New York: McGraw-Hill, 1952, pp 452-467.

...if you can find this book.

Also good, but written from a modern perspective is the 1993 McGraw-Hill *Video Engineering* by a retired RCA Division head and CEO type, Andrew F. Inglis. ISBN 0-07-031716-X