BAC contigs are constructed by aligning BAC regions of known overlap via a variety of methods. One common strategy is to use sequence-tagged site (STS) content mapping to detect unique DNA sites in common between BACs.  The degree of overlap is roughly estimated by the number of STS markers in common between two clones, with more markers in common signifying a greater overlap. Because this strategy provides only a very rough estimate of overlap, restriction digest fragment analysis, which provides a more precise measurement of clone overlap, is often used.  In this strategy, clones are treated with one or two restriction enzymes and the resulting fragments separated by gel electrophoresis.  If two clones, they will likely have restriction sites in common, and will thus share several fragments.  Because the number of fragments in common and the length of these fragments is known (the length is judged by comparison to a size standard), the degree of overlap can be deduced to a high degree of precision.