Proof of the execution of an instrument, when not acknowledged, may be made either:

1. (1) By the party executing it, or either of them;
2. (2) By a subscribing witness; or
3. (3) By other witnesses, in cases mentioned in §§ 18-4-19 and 18-4-20, relating to proof of handwriting.

Source: CivC 1877, § 662; CL 1887, § 3283; RCivC 1903, § 976; RC 1919, § 582; SDC 1939, § 51.1610.