  We investigate the Lyth relationship between the tensor-scalar ratio, r, and
the variation of the inflaton field, Delta phi, over the course of inflation.
For inflationary models that produce at least 55 e-folds of inflation, there is
a correlation between r and Delta phi as anticipated by Lyth, but the scatter
around the relationship is huge. However, for inflationary models that satisfy
current observational constraints on the scalar spectral index and its first
derivative, the Lyth relationship is much tighter. In particular, any
inflationary model with r > 10^-3 must have Delta phi > m_pl. Large field
variations are therefore required if a tensor mode signal is to be detected in
any foreseeable cosmic microwave background (CMB) polarization experiment.
