Sabine's gull treated as comprising a monotypic genus; it is only placed within the genus "Larus" when the genus is enlarged. The black bill and notched tail are almost unique within the gulls, as they are shared only with the swallow-tailed gull of the Galapagos. On the basis of this the two species were often thought to be each other's closest relatives, a hypothesis ruled out by a number of behaviour and ecological differences. Mitochondrial DNA studies confirmed that they are not closely related, and the closest relative of the Sabine's gull is now thought to be the ivory gull, another Arctic
Four-letter word category includes words that are euphemistic evasions of "stronger" words, as well as those that happen to be short and have both an expletive sound to some listeners as well as a sexual or excretory meaning (many also have other, non-vulgar meanings): "" (US), "", "", "" (US), "" (UK, NZ, AUS), "" and "", as several examples. Finally, certain four-lettered terms with limited usage can be considered offensive by some, within the regional dialect in which they are used, such as "" and "mary". Occasionally the phrase "four-letter word" is humorously used to describe common words composed of four