Let K be a tame knot with irreducible exterior M(K) in a
closed, connected, orientable 3–manifold Σ such that
π1(Σ) is cyclic. If ∞ is not a strict
boundary slope, then the diameter of the set of strict boundary
slopes of K, denoted dK, is a numerical invariant of K.
We show that either (i) dK≥2 or (ii) K is a generalized
iterated torus knot. The proof combines results from Culler and Shalen
[Comment. Math. Helv. 74 (1999) 530–547] with a result about the
effect of cabling on boundary slopes.