Lanthanides and actinides together constitute

There are two blocks of rare earths, the lanthanide series and the actinide series. Lanthanum and actinium are both located in group IIIB of the table. When one looks at the periodic table, one notices that the atomic numbers make a jump from lanthanum (57) to hafnium (72) and from actinium (89) to rutherfordium (104). If one skips down to the bottom of the table, one can follow the atomic numbers from lanthanum to cerium and from actinium to thorium, and then back up to the main body of the table. Some chemists exclude lanthanum and actinium from the rare earths, considering the lanthanides to start following lanthanum and the actinides to start following actinium.