COCI, the OpenCitations Index of Crossref open DOI-to-DOI citations **Author(s) **Ivan Heibi – ivan.heibi2@unibo.it Digital Humanities Advanced Research Centre (DHARC), Department of Classical Philology and Italian Studies, University of Bologna, Bologna, Italy Silvio Peroni – silvio.peroni@unibo.it Digital Humanities Advanced Research Centre (DHARC), Department of Classical Philology and Italian Studies, University of Bologna,