In addition to the rules outlined above, Old English poetry had constraints limiting the lengths of verses and the number and distribution of lifts and dips. However, there is still no consensus on what precisely the constraints were, and they must have varied slightly from one poem to another. The most widely used system for classifying Old English prosodic patterns is based on that developed by Eduard Sievers and extended by Alan Joseph Bliss. Sievers' system is a method of categorization rather than a full theory of meter. It does not, in other words, purport to describe the system the "scops" actually used to compose their verse, nor does it explain why certain patterns are favored or avoided. Sievers divided verses into five basic types, labeled A–E. The system is founded upon accent, alliteration, quantity of vowels, and patterns of syllabic accentuation. Another popular system is that of Geoffrey Russom, which is predicated on a theory of meter involving two metrical feet per verse. Another is that of Thomas Cable, based on the idea that each verse contains four syllables, with specific rules for the addition of extra unstressed syllables.