[Chimera-users] On pdb structure match or align

Dear All, With chimera I open A.pdb abd B.pdb. A.pdb contains domain a and doain b, B.pdb contains domain b and domain c, which is to say, the 2 structures share a common domain b. Will you please tell me the chimera specific command to align or match the 2 structures based fully on the common domain b in each structure, without consideration of the domain a and domain c for calculation of the alignment? Best wishes, Smith