The atomic arrangement of a natural rare-earth-rich titanite and two synthetic rare-earth-doped titanites have been refined in space group A2/a, and the atomic arrangement of an undoped P2 1 /a synthetic titanite was also refined for comparison. Previous work has shown that titanite possesses a domain structure, with domains formed of like-displaced Ti atoms in the  octahedral chains. P2 1 /a titanite results when the crystal is formed of a single domain, but as Ti-reversal sites occur in the octahedral chain the apparent A2/a structure results from the average of antiphase domains. Antiphase boundaries occur at O1, which is alternately overbonded or underbonded at the boundaries, depending on the displacement of the neighboring Ti atoms. Type 2 antiphase boundaries exist where two Ti atoms are displaced away from the intervening O1 atom and are energetically unfavorable because of underbonding of that O1 atom. However, substitution of a trivalent rare earth element in the adjacent Ca (super 2+) site relieves that under-bonding, favoring the creation of type 2 antiphase boundaries and stabilization of the A2/a dimorph. The results of high-precision crystal structure analyses demonstrate that rare earth substituents for Ca stabilize the A2/a dimorph at lower substitution levels than required for octahedral substitutions.