Whatever names may be chosen to denote the metrical forms, the _measure_ or _foot_ always remains the unity which is the basis of all modern metrical systems, and of all investigation into metre. For a line or verse is built up by the succession of a limited number of feet or measures, equal or unequal. With regard to the limit of the number of feet permissible in a line or verse, no fixed rule can be laid down. In no case must a line contain more feet than the ear may without difficulty apprehend as a rhythmic whole; or, if the number of feet is too great for this, the line must be divided by a _pause_ or _break_ (caesura) into two or more parts which we may then call rhythmical _sections_. This break is a characteristic mark of the typical Old English alliterative line, which is made up of two rhythmical sections. The structure of this verse was at one time obscured through the practice of printing each of these sections by itself as a short line; but Grimm's example is now universally followed, and the two sections are printed as parts of one long line.[10] Before entering into a detailed consideration of the alliterative long line, it will be needful to make a few general remarks on rhyme and its different species. § =10.= Modern metre is not only differentiated from metre of the classical languages by the principle of _accent_ as opposed to _quantity_; it has added a new metrical principle foreign to the ancient systems. This principle is Rhyme. Instances of what looks like rhyme are found in the classical poets from Homer onwards, but they are sporadic, and are probably due to accident.[11] Rhyme was not in use as an accessory to metre in Latin till the quantitative principle had given way to the accentual principle in the later hymns of the Church, and it has passed thence into all European systems of metre. In our poetry it serves a twofold purpose: it is used either simply as an ornament, or as a tie to connect single lines into the larger metrical unity of stanza or stave, by the recurrence of similar sounds at various intervals. In its widest sense rhyme is an agreement or consonance of sounds in syllables or words, and falls into several subdivisions, according to the extent and position of this agreement. As to its position, this consonance may occur in the beginning of a syllable or word, or in the middle, or in both middle and end at the same time. As to its extent, it may comprehend one or two or more syllables. Out of these various possibilities of likeness or consonance there arise three chief kinds of rhyme in this wide sense, alliteration, assonance, and end-rhyme, or rhyme simply in the more limited and usual acceptation of the word.