Scanning tunneling microscopy has been used to investigate the formation of the 2 x n vacancy line structure on Si(001). We find that quenching the surface from high temperatures results in the formation of vacancies. After further quenching, these vacancies nucleate into chains running perpendicular to the dimer rows. Finally, the vacancy chains connect and develop into vacancy lines which extend for many thousands of Angstrom’s. Each vacancy line consists of mainly two types of dimer vacancies : (1) a di-vacancy and (2) a combination of a single vacancy and a di-vacancy separated by an isolated dimer. All the vacancy lines together with the dimer rows form a 2 x n structure with 6 less than or equal to n less than or equal to 12. Calculations using the Stillinger-Weber potential support the view that the formation of the vacancy line structure is due to the interaction between vacancies.