furlong

The racecourse is where the furlong still earns its keep. British and Irish race distances are posted in miles and furlongs, American tracks set a marker pole at every furlong, and the clocker's benchmark is arithmetic anyone can check: a horse running a furlong in 12 seconds is moving at $201.168/12 \approx 16.8$ m/s, a shade over 60 km/h, which is what a serious workout looks like. A "half in 48" on a training report is four of those 12-second furlongs in a row.

The unit also explains the shape of things that outlived it. An acre is one furlong by one chain, the mediaeval ploughing strip, so the furlong sits inside every land unit this catalog carries: eight to the mile, ten chains long, forty rods. It survives underfoot too. A standard Chicago city block is 660 feet, one furlong, eight blocks to the mile, because the grid was set out with surveyors' chains; a Chicagoan quoting distances in blocks is unknowingly quoting furlongs.

For a body-scale anchor, a 400 m running track is very nearly two furlongs around, and a furlong is about two football pitches laid end to end. The exactness of the metre value comes from the 1959 international yard; nothing about the furlong has been measured since.