  We investigate the prior dependence of constraints on cosmic tensor
perturbations. Commonly imposed is the strong prior of the single-field
inflationary consistency equation, relating the tensor spectral index nT to the
tensor-to-scalar ratio r. Dropping it leads to significantly different
constraints on nT, with both positive and negative values allowed with
comparable likelihood, and substantially increases the upper limit on r on
scales k = 0.01 Mpc^-1 to 0.05 Mpc^-1, by a factor of ten or more. Even if the
consistency equation is adopted, a uniform prior on r on one scale does not
correspond to a uniform one on another; constraints therefore depend on the
pivot scale chosen. We assess the size of this effect and determine the optimal
scale for constraining the tensor amplitude, both with and without the
consistency relation.
