Fillet weldA weld of triangular cross-section joining two surfaces at an angle. Here in a T joint.
FaceThe outer surface of the weld, the side you see. It can be flat, concave or convex.
RootThe deepest point of the weld, in the corner opposite the face.
Base metalThe metal pieces being joined, also called the parent metal.
The fillet weld: 7 4
ToesThe two edges where the face of the weld meets the plates.
LegsThe two sides of the weld along the plates. The leg length is usually given as the weld's size.
ThroatThe distance from the root to the middle of the face. It is usually at least as thick as the metal being welded.
Heat-affected zoneMetal next to the weld that did not melt. It was changed by the heat of welding.
The groove weld: 7 5
Groove weldWeld metal filling the groove between two plates of a butt joint.
ReinforcementWeld metal that stands above the surface of the plates. It is not counted in the weld's thickness.
BevelThe angled edge cut on a plate to make the groove. In a V groove both edges are bevelled.
Groove angleThe whole angle between the two bevels. It is kept as small as still gives the welder access.
Root openingThe gap between the plates at the joint's root.
How this list was compiled: Read 2026-09-27. (1) Wikipedia, Fillet weld, https://en.wikipedia.org/wiki/Fillet_weld : five parts, the root (the deepest penetration, opposite the hypotenuse), the toes (the edges of the hypotenuse), the face (the outer visible side, concave, flat or convex), the legs (the other two sides; the leg length is usually the size of the weld) and the throat (from the centre of the face to the root, usually at least as thick as the metal welded). (2) Wikipedia, Welding joint, https://en.wikipedia.org/wiki/Welding_joint : butt joints and grooves; single-V welds have both edges bevelled; the opening or gap at the root and the included angle of the groove are chosen to need the least weld metal that still gives access; a butt weld's thickness does not count the weld reinforcement; base metals. (3) Wikipedia, Heat-affected zone, https://en.wikipedia.org/wiki/Heat-affected_zone : base material that is not melted but has had its properties altered by the heat of welding. Licence: own work
The drawing: Drawn for this site by script, 2026-09-27. Own work, nothing traced. Licence: own work
How it works
These are the parts of a weld on two welds cut across: a fillet weld in a T joint, then a V-groove butt weld. Start with the 4 main parts.
Read the full guide
A fillet weld
A fillet weld is a triangle in a corner. Its face is the outer side and the toes are where the face meets the plates. The legs run along the plates, the root is the deepest point, and the throat runs from root to face.
A groove weld
For a butt weld the plate edges are bevelled to make a V. The groove angle is the angle across it and the root opening is the gap at the bottom. Weld standing above the plates is reinforcement.
Picture a fillet weld as a triangle: two legs along the plates, the face across them, a toe at each end, the root inside.
Questions and answers
What are the five parts of a fillet weld?
The root, the toe, the face, the leg and the throat. The root is the deepest point, the toes are the edges of the face, and the throat runs from the root to the middle of the face.
How is the size of a fillet weld measured?
Usually by its leg length, the length of the weld's sides along each plate.
What is the heat-affected zone?
The metal beside a weld that did not melt but was changed by the heat of welding. Its structure and properties are no longer quite those of the rest of the metal.