The limits and effect on cell size of silver substitution into synthetic tetrahedrite, Cu₁₀(Zn,Fe,Cd₂) Sb₄S₁₃, are investigated for comparison with natural tetrahedrite. The limit of Ag substitution into natural zincian tetrahedrite is ∼ 4 atoms per half unit cell and into iron tetrahedrite ∼ 6.5 atoms (with rare exceptions). The cell size of natural tetrahedrite increases with increasing Ag content up to 4 atoms but decreases with further Ag substitution. The highest Ag substitution achieved in synthetic tetrahedrites was 4.7 atoms in Zn₂-tetrahedrite, 7.02 atoms in Cd₂-tetrahedrite and 6.80 atoms in Fe₂-tetrahedrite. The cell size of synthetic tetrahedrites shows a continuous increase with increasing Ag content, the largest cell size of a = 10.927 Å being in the Cd₂-tetrahedrite with 7.02 atoms Ag. The iron content of tetrahedrite systematically increased from 1.1 to 2.0 atoms per half unit cell tetrahedrite with increase from 0 to ∼ 4 atoms Ag. The different limit of Ag substitution between Zn₂ and Cd₂ tetrahedrite can be explained by size constraints on the expanding structure. An explanation is given for a limit of 7 atoms Ag substitution in tetrahedrite using a combined electron band/molecular orbital approach.