Survey of η,δ parameter space for solutions of the K type with f_pms = 1 (Eq. (4)) using the canonical pre-main-sequence data. Open circles and small dots delineate models consistent with the data at the 1 and 5 per cent confidence level or better, respectively, whereas the thick solid dot shows a model consistent at the 50 per cent level or better. The K1 IPF is shown as the thick cross. Note that the K1 IPF is not the same as the best solution found here. This is unsurprising because the K1 IPF was derived by constraining the IPF with both the pre-main-sequence and main-sequence period distribution functions. The best solution found here thus lacks one major constraint leading to the K1 IPF, but both are sufficiently close to conclude that the K1 IPF is a good solution to the pre-main-sequence data alone. The straight line shows the asymptotic solution for large δ: f_K1(lP) = (η/δ)   (lP − lP_min), with η/δ = 0.0339.