A contig, derived from the word contiguous (sharing a common border or next to each other in a sequence), is a small piece of DNA that when layered together, overlap with other small pieces of DNA. The over lapping sequences from multiple reads form a consensus sequence allowing for the assembly of large pieces of DNA (like the genome).

Because DNA sequencing currently requires smaller pieces of DNA, the strand to be sequenced is amplified, the amplifications are randomly sheared into many small pieces (300-1000bp) and reassembled by a computer program to produce the sequence of the entire strand.