Keeled scales

Keeled scales refer to reptile scales that, rather than being smooth, have a ridge down the center that may or may not extend to the tip of the scale, making them rough to the touch. According to Street's (1979) description of European lizards and snakes, in those that have keeled scales the keels are usually stronger in male specimens and are consistently arranged according to the species of reptiles, even though many others do not have them. With European lizards, the dorsal scales are usually well keeled, while those on the flanks are more weakly keeled and those on the belly smooth.
Klauber (1997), describes how the keels on the scales of "Crotalus" rattlesnakes are particularly strong mid-dorsally, but gradually weaken on the lateral rows with the ventral scales being smooth. He stated that the function of the keels was uncertain, but that since they reduced shininess, and thus made it easier for the animals to conceal themselves, it afforded them an evolutionary advantage.
In some viperines, most notably those of the genus "Echis", the lateral scales are not only keeled, but the keels have minute serrations. The snakes use this in a warning and threat display: the body is arranged into a series of parallel C-shaped (counterlooped) coils that they rub together continuously to produce a sizzling sound, rather like water on a hot plate. This method of making sound is called stridulation.