Keeled scales refer to reptile

Reptile

Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

 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 lizard

Lizard

Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...

s and snake

Snake

Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...

s, 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 rattlesnakes

Crotalus

Crotalus is a genus of venomous pit vipers found only in the Americas from southern Canada to northern Argentina. The name is derived from the Greek word krotalon, which means "rattle" or "castanet", and refers to the rattle on the end of the tail which makes this group so distinctive...

 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.

Keeled scales scatter light differently when compared to smooth, flat scales. As a result, most reptiles with keeled scales are not shiny: they have a dull, non-reflective appearance. It is therefore possible that keeled scales further improve the camouflage effects already provided by skin coloration/shading and disruptive patterns.

In some vipers

Viperinae

The Viperinae, or viperines, are a subfamily of venomous vipers found in Europe, Asia and Africa. They are distinguished by their lack of the heat-sensing pit organs that characterize their sister group, the Crotalinae. Currently, 12 genera and 66 species are recognized...

, most notably those of the genus Echis

Echis

Echis is a genus of venomous vipers found in the dry regions of Africa, the Middle East, Pakistan, India and Sri Lanka. They have a characteristic threat display, rubbing sections of their body together to produce a "sizzling" warning sound...

, the lateral scales are not only keeled, but the keels have minute serrations. The snakes use this in a warning/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

Stridulation

Stridulation is the act of producing sound by rubbing together certain body parts. This behavior is mostly associated with insects, but other animals are known to do this as well, such as a number of species of fishes, snakes and spiders...

.