Reed, Bruce (1999), "Introducing directed tree width", Electronic Notes in Discrete Mathematics, vol. 3, Elsevier, pp. 222–229, doi:10.1016/S1571-0653(05)80061-7
Johnson, Thor; Robertson, Neil; Seymour, Paul; Thomas, Robin (2001), "Directed Tree-Width", Journal of Combinatorial Theory, Series B, vol. 82, pp. 138–154, doi:10.1006/jctb.2000.2031