The Vaishya may take two wives. In both of them, he takes birth himself. The Shudra can take only one wife, from his own caste. The son begotten by him upon her becomes a Shudra.
द्वो चापि भार्ये द्वयोरात्मास्य जायते। शूद्रा शूद्रस्य यचाप्येका शूद्रमेव प्रजायते॥