  We investigate relations between different width parameters of graphs, in
particular balanced separator number, treewidth, and cycle rank. Our main
result states that a graph with balanced separator number k has treewidth at
least k but cycle rank at most k(1 + log (n/k)), thus refining the previously
known bounds, as stated by Robertson and Seymour (1986) and by Bodlaender et
al. (1995). Furthermore, we show that the improved bounds are best possible.
