Hello sachit_baroda, As professor Crisi stated, the root of the weld is the basic building block of any welding joint. In some cases the root could be exposed on the opposite side of a welded joint, such as is the case with a full penetration open-root joint. In other cases the root could be tied into a backing strip where full penetration joints utilize backing strips to aid in forming the root weld and tying the two parts together and supporting the welding filler metal as it is applied. In the case of fillet welds the root weld could describe the initial pass for multi-pass welds used on certain lap-joint, T-joint, socket weld, or other similar type weld configurations. On a single pass fillet weld it would describe the point where the weld bead actually ties the two members to one another. The root has to have complete penetration to the corner of the joint so as not to allow voids, or to trap slag or allow other types of discontinuities to be presents as these could lead to weld failure. Where the root is exposed in items such as the I.D. of piping systems, tanks, or other vessels, it is important that the root is properly formed and void of areas of undercut, underfill, overfill, and incomplete penetration. Any of the previosly mentioned items and a number of others could lead to failure of a welded joint. I am sure others can expand on this explanation. Just my $.02 worth. Best regards, aevald