Final answer:
A weld applied to the other side of a joint before the primary groove weld is started is known as a backing weld and provides support for a higher-quality groove weld.
Step-by-step explanation:
A weld applied to the other side of a joint before the primary groove weld is started is known as a backing weld or back weld. This weld is used to support and ensure the complete fusion of the subsequent groove weld, especially when the joint is subjected to high loads or when the weld quality is critical for the application. The backing weld serves as a foundation to prevent melting through when performing the groove weld and aids in achieving a clean, sound joint with the required mechanical properties.