Proof by parol evidence, §24-3-7. Parol evidence. Generally, §§24-3-1 to 24-3-10. Performance. Changing place. Parol evidence, §24-3-6. Enlarging time. Parol evidence, §24-3-6. Subsequent agreements. Proof by parol evidence, §24-3-6. Surrounding circumstances. Evidence. Admissibility to aid in construction of contracts, §24-3-4. Usage. Known and established usage. Admissible in aid of construction, §24-3-5. Void and illegal contracts.