The sailing frigates of the United States built from 1797 on were unique in that their framing was made of American live oak, a particularly hardy genus that made very resilient hulls; as a result of this, the ships were known to withstand damage that would have scuppered frigates of other nations. American frigates were also very heavily armed; the USN's 44s carried 24-pound cannon as opposed to the 18-pounders usual in frigates, and like most ships of the period carried more than their nominal rate, 56 guns or more. On the other hand, the USN classed ships with 20 to 26 guns as "third-class frigates", whereas the Royal Navy did not.