What specific structural characteristic defines the modern Wheaten Terrier's coat regarding underlayers?

It is a single coat that lacks an undercoat.

The physical structure of the Soft-Coated Wheaten Terrier's coat is a crucial element linking its past utility to its modern presentation. Uniquely among many breeds, the Wheaten coat is classified as a single coat, meaning it completely lacks the insulating layer typically found beneath the outer guard hairs of many other breeds. This absence of an undercoat contributes significantly to the characteristic texture of the coat but also results in minimal shedding, which is beneficial for contemporary households. In contrast, the early working dogs likely required a tougher, somewhat wiry coat built for harsh environmental resistance and rough physical work. The modern standard emphasizes a soft, silky texture, a transition that highlights the dog's journey away from rugged field labor toward domestic companionship.