My thought - may be to do with octave etc, lines on sheet music La,la.LA!
well spotted, even I can not find "tave" in dictionary ... no idea where I saw it ... I have probably made it up
anyway, I meant rungs
Possibly from an old poem - i have looked all over, Including Oh captain my captain... nor the Rime of the Ancient mariner
edit for rung looks good - hope to fin tave somewhere other than t'ave (to have)