It's an old nautical expression that predates aviation; often on old sailing ships you find a gold cheat line painted round the black hull just under the deck.

Called a "cheat" line because a horizontal line "cheats" or "tricks" the eye into thinking the object it is applied to is longer/sleeker than it actually is - which is why airlines use it in their colour schemes, of course.

Not as universal in colour schemes as it once was - in the early jet years, for example, most schemes had a coloured cheat line round the windows, white above (to reflect the sun - it reduces the load on the air conditioning considerably), natural aluminium below, and a tame logo on the fin. The white and the aluminium areas were also effectively horizontal stripes that gave the same effect.

Some old liveries even had horizontal "speed stripes" for the same purpose. And for those who recall the old American livery from the 1930s to the 1960s (or have seen the couple of their current aircraft nicely done in the old-style livery), with the horizontal red "lightning flash" along the aircraft, that again gave the same illusion to the eye.

[This message has been edited by WHBM (edited 04-03-2003).]