Change . At first I was only partially right, but GoogleBooks knew. He pointed me to A Beginner Book of TeX Raymond Serul, Silvio Levy, Silvio Vieira Ferreira Levy, which says:
Three variables control this behavior \baselineskip, \lineskipand \lineskiplimit.
Separation of lines by default \baselineskip, but not allowed to close the fields below \lineskiplimit. If they were, the vertical distance would be adjusted until \lineskipit was between them ...