We prove that every
monotone basis (decomposition) for Lp(μ),1 < p < ∞, is unconditional. The
structure of such bases is closely related to that of the usual Haar basis. This
structure is described here, and it is shown that there is an uncountable number of
mutually non-equivalent monotone bases for Lp. The structure of monotone bases in
L1 is also considered, and the equivalence question there is characterized in analytic
terms.