Whereas documents in a bilingual corpus are typically very closely aligned, monolingual corpora exhibit a much looser level of alignment, with similar content expressed using widely divergent wording, grammatical form, and sentence order.
many of the simple surface-based methods that have proven to be so successful in bilingual sentence alignment, such as correlation of sentence length, linearity of alignment, and a predominance of one-to-one sentence mapping, are much less likely to be effective for monolingual sentence alignment.