Furthermore, Whorf introduced the concept of the allophone, a word that describes positional phonetic variants of a single superordinate phoneme; in doing so he placed a cornerstone in consolidating early phoneme theory. The term was popularized by G. L. Trager and Bernard Bloch in a 1941 paper on English phonology and went on to become part of standard usage within the American structuralist tradition. Whorf considered allophones to be another example of linguistic relativity. The principle of allophony describes how acoustically different sounds can be treated as reflections of a single phoneme in a language. This sometimes makes the different sound appear similar to native speakers of the language, even to the point that they are unable to distinguish them auditorily without special training. Whorf wrote that: "are also relativistic. Objectively, acoustically, and physiologically the allophones of [a phoneme may be extremely unlike, hence the impossibility of determining what is what. You always have to keep the observer in the picture. What linguistic pattern makes like is like, and what it makes unlike is unlike".(Whorf, 1940)