Cairns are considered the oldest terrier breed in Scotland, the base root from which other breeds such as the Skye terrier, Scottish terrier and West Highland White terrier were bred. Scotland (and I've been there!) has a WET, COLD, unforgiving climate. As this breed developed, only those dogs which could survive outside living, year round, in this most unforgiving of environments survived to breed. And this included whelping pups too; there were no heat lamps, bottle feeding and vaccinations! The tough, rugged pups survived.
The cairn is today primarily a pet, however, the goal of a reputable breeder is to produce dogs which conform closely to the written "standard" of how a historical cairn should be. A functional cairn.
One of the most important aspects of a cairn is a proper working terrier coat. What defines a proper coat, or "jacket" as the hunting folk call it?
A proper jacket will afford protection to the terrier as it runs through dew covered fields, crosses brooks, ploughs through snow and works game in the cold, damp dirt underground. Most important of all attributes is that it be "waterproof". This is accomplished by having a very hard, stiff, outer coat which completely covers a soft, dense undercoat. The coat should feel harsh to the touch and lay flat enough to keep driving rain out.
The biggest fault a working terrier like a cairn can have is what is called an "open" coat; a soft, fluffy, silky coat of hair that allows water to soak to the skin, ice to hang on the coat and offers little protection from the bites of vermin.
The AMERICAN KENNEL CLUB describes a correct Cairn terrier coat:
Coat: Hard and weather-resistant. Must be double-coated with profuse harsh outer coat and short, soft, close furry undercoat.
Here are the faults the AKC lists for cairn terrier coat:
Coat: Open coats, blousy coats, too short or dead coats, lack of sufficient undercoat, lack of head furnishings, lack of hard hair on the legs. Silkiness or curliness.
The KENNEL CLUB of Scotland/UK, where the breed originated, described the breed as:
The Cairn is remarkably unchanged in appearance throughout the centuries, lighter in build than the other terrier breeds from Scotland but a game, rugged and harsh coated sporting terrier. Agile, alert, of workmanlike, natural appearance. Standing well forward on forepaws. Strong quarters. Deep in rib, very free in movement. Weather-resistant coat. Very important: weather-resistant. Must be double-coated, with profuse, harsh, but not coarse, outer coat; undercoat short, soft and close. Open coats objectionable. Slight wave permissible.
Unfortunately, show ring fads are hard for dog show people to resist. If professional handers are presenting top dogs with a specific "look", and winning, it won't be long until everyone is trying that look in hopes of winning.
Currently there is a fad for a soft, open coat, which can stand up and away from the body, especially around the head, and can then be trimmed and shaped to alter the outline of the dog, which the skilled handler/groomer can use to hide faults.