The study contains quantitative conditions and limitations of the use of butt, fillet and butt-fillet welds in modern welded constructions. Examples of improper use are given. Calculated loss of material and capacity structures associated with the use of fillet welds with a convex face, their asymmetry, as well as the benefits of in-depth penetration obtained by hybrid (plasma-MAG, laser-MAG) or other, leading to the achievement of butt welds and butt-fillet welds with high-efficiency welding technologies. A list of the requirements for the correct use of fillet welds and features differing from butt, butt-fillet and fillet welds is given.