  More than a century ago, Solon Bailey's pioneering investigations of the
variable stars in globular clusters allowed the first period-amplitude diagrams
to be constructed for their RR Lyrae stars. These diagrams differ from cluster
to cluster, and there has been debate as to whether these differences are
correlated mainly with [Fe/H] or with Oosterhoff type. It is clear now that a
cluster's Oosterhoff type plays an important role in determining its
period-amplitude relation, although the Oosterhoff dichotomy itself is
correlated with metallicity. Not all clusters follow the usual patterns,
however. The globular clusters NGC 6388 and NGC 6441 have period-amplitude
diagrams similar to those of metal-poor Oosterhoff type II globular clusters,
but they themselves are comparatively metal-rich. The period-amplitude diagrams
of Oosterhoff-intermediate systems are discussed.
