I have seen many types of periodic tables which I gather into 3 categories based on how they handle the Lanthanide series (and the Actinide series is handled in an analogous way). The tables do not say the things I say below, but they imply it by the layout. 1. Lanthanum is in group 3, but not in the Lanthanide series. The Lanthanide series includes Cerium through Lutetium. The Lanthanide series is not in any group. ex: http://periodic.lanl.gov/default.htm 2. Lutetium is in group 3, but not in the Lanthanide series, The Lanthanide series includes Lanthanum through Ytterbium. The Lanthanide series is not in any group ex: http://www.webelements.com/webelements/scholar/ 3. The Lanthanide series includes Lanthanum through Lutetium. The entire Lanthanum series is in group 3. ex: http://en.wikipedia.org/wiki/Periodic_table_(standard) Can we say that one of these correctly depicts theory? Or is it a case of differing, but defensible points of view?