Paired-end tags (PET) (sometimes "Paired-End diTags", or simply "ditags") are the short sequences at the 5’ and 3 ends of a DNA fragment which are unique enough that they (theoretically) exist together only once in a genome, therefore making the sequence of the DNA in between them available upon search (if full-genome sequence data is available) or upon further sequencing (since tag sites are unique enough to serve as primer annealing sites). Paired-end tags (PET) exist in PET libraries with the intervening DNA absent, that is, a PET "represents" a larger fragment of genomic or cDNA by consisting of a short 5 linker sequence, a short 5 sequence tag, a short 3 sequence tag, and a short 3' linker sequence.  It was shown conceptually that 13 base pairs are sufficient to map tags uniquely. However, longer sequences are more practical for mapping reads uniquely. The endonucleases (discussed below) used to produce PETs give longer tags (18/20 base pairs and 25/27 base pairs) but sequences of 50–100 base pairs would be optimal for both mapping and cost efficiency. After extracting the PETs from many DNA fragments, they are linked (concatenated) together for efficient sequencing. On average, 20–30 tags could be sequenced with the Sanger method, which has a longer read length. Since the tag sequences are short, individual PETs are well suited for next-generation sequencing that has short read lengths and higher throughput. The main advantages of PET sequencing are its reduced cost by sequencing only short fragments, detection of structural variants in the genome, and increased specificity when aligning back to the genome compared to single tags, which involves only one end of the DNA fragment.