  "Citation classics" are not only highly cited, but also cited during several
decades. We test whether the peaks in the spectrograms generated by Reference
Publication Years Spectroscopy (RPYS) indicate such long-term impact by
comparing across RPYS for subsequent time intervals. Multi-RPYS enables us to
distinguish between short-term citation peaks at the research front that decay
within ten years versus historically constitutive (long-term) citations that
function as concept symbols (Small, 1978). Using these constitutive citations,
one is able to cluster document sets (e.g., journals) in terms of
intellectually shared histories. We test this premise by clustering 40 journals
in the Web of Science Category of Information and Library Science using
multi-RPYS. It follows that RPYS can not only be used for retrieving roots of
sets under study (cited), but also for algorithmic historiography of the citing
sets. Significant references are historically rooted symbols among other
citations that function as currency.
