The Five Joints
Every welded assembly reduces to five joint geometries, defined by how the two pieces meet — before any weld is placed. · 11 min
Set the gun down for a folio. Before a weld exists, two pieces of steel have to meet, and the way they meet is the joint — a fact of geometry, independent of whatever weld you later put on it. Fabricators the world over reduce every meeting to a short list, and reading any drawing, symbol, or weld procedure starts with recognizing which one you are looking at.
Guess before you learn
Two pieces of metal can meet in only so many geometrically distinct ways. How many joint types does the American Welding Society count?
Five. Butt, lap, tee, corner, edge — every assembly you will ever tack together is built from these. If you guessed higher, you were probably counting welds or positions; those multiply, but the meetings themselves do not.
9–12
3–5
Five ways, five names. Edge to edge in a line: butt. One lying on top of the other: lap. One standing on the middle of the other: tee. Two edges meeting to make an L: corner. Two lying face to face with their edges together: edge.
6–8
The definitions turn on two questions: how do the pieces' planes relate, and where do they touch? Butt — same plane, edge to edge. Lap — parallel planes, faces overlapping. Tee — perpendicular planes, one piece's edge meeting the other's face away from its edge. Corner — perpendicular planes, edge meeting edge at roughly 90 degrees.
Edge — parallel planes, faces together, joined along the matched edges. And keep the ledger straight: the joint is the meeting; the weld is the metal you add to it. The next folio maps welds onto joints, because the same tee can carry several different welds.
9–12
Why five and not fifty: the classification ignores size, exact angle, and edge preparation, keeping only the essential geometry of the meeting. A 60-degree corner is still a corner joint; a beveled butt is still a butt. That abstraction is what lets one symbol system — folio 14 — describe every assembly a shop will ever build.
Each joint also implies where a weld can physically go. A butt offers the seam itself; a lap offers a fillet along each overlapping edge; a tee offers both sides of the upright; a corner offers inside, outside, or both; an edge offers only the matched rim.
K–2
Take two flat blocks. Touch them edge to edge in a line. Stack one on top of the other. Stand one up on the middle of the other, like a wall on a floor.
Meet their corners to make an L. Or hold them flat together, side by side, edges touching. Five ways — that is all of them.
Undergrad
Joints differ in how they carry load. A butt joint keeps force in line — stress flows straight through the splice, which is why pipelines and pressure vessels are butt-welded. A lap joint is eccentric: the offset between the two plates' midplanes turns pure tension into tension plus bending at the weld.
Tee and corner joints concentrate stress where the weld toe meets the upright, and an edge joint fuses only the rim — the unfused faces act as a built-in notch, which is why edge joints are kept off load paths. Joint selection is structural design before it is welding.
Postgrad
AWS A3.0 fixes the taxonomy: five joint types defined by member position, distinct from weld types and from groove geometry — separate axes of one specification space. Fatigue codes then discriminate hard among them: a ground butt splice earns a high fatigue class; a load-carrying fillet on a lap joint sits near the bottom of the table.
The mechanics behind that table: lap eccentricity adds a secondary bending moment proportional to the plate offset; unfused lands in edge and partial-penetration joints behave as embedded cracks under opening load; and toe geometry sets the stress-concentration factor governing initiation life. Geometry chosen at fit-up decides more than any parameter set at the machine.
joint
The geometry of how two pieces meet — named before, and independent of, any weld placed on it.
Why is this true?
Why is the joint named before the weld?
Because the meeting exists as a fact of geometry the moment two parts are fitted, while the weld is a choice made about that geometry. One tee joint can carry a fillet on one side, on both sides, or a groove — and the joint's name never changes.
Now attach the five to the world. Butt joints splice plate and pipe into longer plate and pipe — pipelines, tank shells, guardrail. Lap joints rule sheet metal and repair: patch panels, trailer decking, anywhere one piece can ride over another. Tee joints build structure — stiffeners, gussets, a post on a baseplate. Corner joints close boxes: frames, enclosures, weldment cases. Edge joints stay light-duty — sheet-metal rims and covers, never load paths. When you can name the joint from across the shop, drawings start reading themselves.
Name every joint on a steel toolbox — the steps fade as you master them
Corner joint — the box's outside seam
Tee joint — edge into face
Lap joint — face on face
Butt joint — the one that disappears when done well
Five meetings, and you can now name them all from geometry alone. The next folio adds the welds themselves — fillet and groove — and maps them onto these joints, because a tee can be welded three different ways and a drawing has to tell you which. The joint was the question; the weld is the answer.
Practice — new ink and old, interleaved
1.A square tube post welded upright onto a flat baseplate?
2.The steel turns out to be galvanized. Before any defect-hunting, what is the first concern?
3.Why can a sound weld outperform a bolted or glued joint in the same steel?
4.The five joints, from memory, each with its defining geometry.
Butt — edge to edge in one plane; lap — faces overlapping; tee — edge into face at 90 degrees; corner — edges meeting in an L; edge — faces parallel with edges together.
How close were you? Grade yourself honestly — it sets your review date.
5.From memory: the four variables of a stringer bead, with a number for each.
Work angle near 90°; travel angle a 5–15° drag; arc length one rod diameter; travel speed steady — the pace that keeps width at about twice the rod diameter.
How close were you? Grade yourself honestly — it sets your review date.
6.Match the fabrication to its joint.
7.How many of the five joints put the two pieces' faces parallel to each other?