Many verse forms have been used in epic poems through the ages, but each language's literature typically gravitates to one form, or at least to a very limited set. Ancient Sumerian epic poems did not use any kind of poetic meter and lines did not have consistent lengths; instead, Sumerian poems derived their rhythm solely through constant repetition, with subtle variations between lines. Indo-European epic poetry, by contrast, usually places strong emphasis on the importance of line consistency and poetic meter. Ancient Greek and Latin poems were written in dactylic hexameter. Old English, German and Norse poems were written in alliterative verse, usually without rhyme. Italian, Spanish and Portuguese long poems were usually written in terza rima or especially ottava rima. From the 14th century English epic poems were written in heroic couplets, and rhyme royal, though in the 16th century the Spenserian stanza and blank verse were also introduced. The French alexandrine is currently the heroic line in French literature, though in earlier periods the decasyllable took precedence. In Polish literature, couplets of Polish alexandrines (syllabic lines of 7+6 syllables) prevail. In Russian, iambic tetrameter verse is the most popular. In Serbian poetry, the decasyllable is the only form employed.