An informal unit of distance used to measure the distance one horse leads another at the finish of a race. The neck is usually interpreted to be 1/4 length or a little less; this is roughly 2 feet or 0.6 meter.
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}