A heterodimeric protein, comprising (i) a first C H 1 domain (C H 1) and a first C L domain (C L), the first C H 1 and the first C L interacting together at a first C H C L interface to form a first C H C L domain (CHCL); (ii) a second C H 1 domain (C H 1) and a second C L domain (C L), the second C H 1 and the second C L interacting together at a second C H C L interface to form a second C H C L domain (C H C L); wherein the first C H 1 is engineered to differ from the second C H 1 by at least one C H 1 mutant residue in the first C H 1; and the first C L is engineered to differ from the second C L by at least one C L mutant residue in the first C L; such that the C H 1 mutant residue and the C L mutant residue of the first C H C L interact with each other in preference to the corresponding residue positions on the second C H C L, the interacting mutant residues of the first C H 1 and first C L thereby forming a first complementary residue set, and wherein the first C H 1 is attached to a first variable heavy domain (V H), and the first C L is attached to a first variable light domain (V L), and the second C H 1 is attached to a second V H, and the second C L is attached to a second V L, such that when combined, the first V H, first V L, first C H and first C L together form a first Fab, and when combined, the second V H, second V L, second C H 1, and second C L form a second Fab, and wherein preferential formation of the first Fab and the second Fab does not rely on complementary pairing of the variable domains, and characterized in that the first complementary residue set comprises C H 1-124K, C L -176D, C L -133S, and C H 1-190S and wherein the second C H 1 is engineered to differ from the first C H 1 by at least one C H 1 mutant residue in the second C H 1; and the second C L is engineered to differ from the first C L by at least one C L mutant residue in the second C L; such that the C H 1 mutant residue and the C L mutant residue of the second C H C L preferentially interact with each other over the corresponding residue positions on the first C H C L, the interacting mutant residues of the second C H 1 and second C L thereby forming a second complementary residue set, and wherein the mutant residues of the first complementary residue set are different to the mutant residues of the second complementary residue, and wherein the second complementary residue set comprises: C H 1-124E, C L -176K, C H 1-188G; and C L -133S, and wherein the C H 1 numbering and the C L numbering is according to Kabat numbering. (i) a first C H 1 domain (C H 1) and a first C L domain (C L), the first C H 1 and the first C L interacting together at a first C H C L interface to form a first C H C L domain (CHCL); (ii) a second C H 1 domain (C H 1) and a second C L domain (C L), the second C H 1 and the second C L interacting together at a second C H C L interface to form a second C H C L domain (C H C L); wherein the first C H 1 is engineered to differ from the second C H 1 by at least one C H 1 mutant residue in the first C H 1; and the first C L is engineered to differ from the second C L by at least one C L mutant residue in the first C L; such that the C H 1 mutant residue and the C L mutant residue of the first C H C L interact with each other in preference to the corresponding residue positions on the second C H C L, the interacting mutant residues of the first C H 1 and first C L thereby forming a first complementary residue set, and wherein the first C H 1 is attached to a first variable heavy domain (V H), and the first C L is attached to a first variable light domain (V L), and the second C H 1 is attached to a second V H, and the second C L is attached to a second V L, such that when combined, the first V H, first V L, first C H and first C L together form a first Fab, and when combined, the second V H, second V L, second C H 1, and second C L form a second Fab, and wherein preferential formation of the first Fab and the second Fab does not rely on complementary pairing of the variable domains, and characterized in that the first complementary residue set comprises C H 1-124K, C L -176D, C L -133S, and C H 1-190S and wherein the second C H 1 is engineered to differ from the first C H 1 by at least one C H 1 mutant residue in the second C H 1; and the second C L is engineered to differ from the first C L by at least one C L mutant residue in the second C L; such that the C H 1 mutant residue and the C L mutant residue of the second C H C L preferentially interact with each other over the corresponding residue positions on the first C H C L, the interacting mutant residues of the second C H 1 and second C L thereby forming a second complementary residue set, and wherein the mutant residues of the first complementary residue set are different to the mutant residues of the second complementary residue, and wherein the second complementary residue set comprises: C H 1-124E, C L -176K, C H 1-188G; and C L -133S, and wherein the C H 1 numbering and the C L numbering is according to Kabat numbering.