Vowel whose quality does not change over the duration of the vowel is called a monophthong. Monophthongs are sometimes called "pure" or "stable" vowels. A vowel sound that glides from one quality to another is called a diphthong, and a vowel sound that glides successively through three qualities is a triphthong. All languages have monophthongs and many languages have diphthongs, but triphthongs or vowel sounds with even more target qualities are relatively rare cross-linguistically. English has all three types: the vowel sound in "hit" is a monophthong , the vowel sound in "boy" is in most dialects a diphthong , and
National Register of Historic Places property types National Register of Historic Places property types The U.S. National Register of Historic Places (NRHP) classifies its listings by various types of properties. Listed properties generally fall into one of five categories, though there are special considerations for other types of properties which do not fit into these five broad categories or fit into more specialized subcategories. The five general categories for NRHP properties are: building, district, object, site, and structure. Listed properties generally fall into one of five categories, though there are special considerations for other types of properties which do not fit into these five broad categories or