A square tenon will not drop into a pocket milled with a round bit. The leftover radius at each internal corner is the whole problem, and every corner relief is a different answer to the same question: where do you steal that material from?
Three styles cover almost every hobby and shop job — a full corner, a minimal dogbone, and a T-bone. They all use one radius R, half the bit diameter. They differ only in where the arc centre sits, which changes how deep the overcut goes and how much of each wall it eats.
Why a round cutter cannot cut a square corner
An end mill removes a circle of material wherever it sits. Drive that circle into a 90° internal corner and the closest it can get is tangent to both walls, leaving a fillet of radius R in the corner. The pocket looks square in CAM and comes off the machine with four rounded corners.
A square mating part — a tenon, a divider, a drawer bottom, a finger joint — has sharp 90° outside corners. Those corners hit the leftover material before the faces ever touch. The joint stands proud, rocks, or refuses to seat at all. Sanding the tenon corners is the traditional fix and it destroys the fit you machined for.
Corner relief inverts the problem. Instead of making the part fit the pocket, you cut a small extra arc into the pocket so the leftover radius moves out of the mating part's way. The visible cost is a small bite past the corner. The benefit is a joint that closes with hand pressure.
Full corner: the arc centre sits on the vertex
Put the bit centre exactly on the original corner point and you get the most material removal possible at that vertex. The resulting arc is a three-quarter circle, 270°, wrapping around behind the corner. Overcut depth measured along the diagonal is a full R, and each adjacent edge is consumed by R.
This is the "just drill out the corner" move, and it is the most visually aggressive of the three. On a 1/4″ bit that is a 6.35 mm circle biting out of your corner — impossible to miss on an exposed joint.
Use it where the joint is hidden, where the mating part may swell with finish or paint, or where an edge is too short to accept the other styles. It is the cheapest style in edge length: two full corners on one wall need only about 2R of that wall, less than either alternative.
Minimal dogbone: the arc centre sits on the bisector
Slide the centre out along the 45° angle bisector to distance R from the vertex and the circle still clears both walls, but it pokes past the original corner by only R(1 − 1/√2) ≈ 0.293R. For a 1/4″ bit that bulge is about 0.93 mm; for a 1/8″ bit, about 0.47 mm.
That is the whole appeal. Inside a cabinet carcass or a plywood box a sub-millimetre bulge is invisible, and the joint still seats because the leftover corner radius has been relieved.
The trade is edge length. The circle crosses each wall at distance √2·R from the corner, so each minimal dogbone consumes √2·R ≈ 1.414R of every adjacent edge — roughly 41% more than a full corner. Two of them on one wall need 2√2·R plus tolerance, which is 9.03 mm for a 1/4″ bit. This is the generator's default because it is the smallest overcut that still works.
T-bone: the arc centre sits on one wall
Park the centre on one of the two walls, at distance R from the corner, and the circle relieves that corner from a single direction. The result looks like a slot poking sideways out of one wall — hence the name.
Overcut depth is still R, but only into the wall you chose. The facing edge is consumed by 2R; the other edge is consumed by nothing at all. That asymmetry is the entire point. On a narrow slot where one dimension has no room to spare, a T-bone aimed along the long axis leaves the narrow dimension completely untouched.
It is not a prettier dogbone, and choosing it for looks is a mistake. A T-bone facing the wrong wall leaves the tenon corner still touching cutter radius on the unrelieved side, and the joint will not close. If you are unsure, leave the axis on auto.
Edge length is what actually decides
Style choice usually comes down to arithmetic, not taste. For every edge, the two reliefs at its ends must fit without overlapping: edge length must exceed the consumption at the start corner plus the consumption at the end corner plus a small tolerance, which this tool sets at 0.05 mm.
Consumption per corner is R for a full corner, √2·R for a minimal dogbone, 2R or 0 for a T-bone depending on which way it faces, and 0 for a disabled corner. If two reliefs overlap, the intersections swap order and the rewritten contour self-intersects — a shape CAM will either reject or, worse, cut as something you did not draw.
The generator refuses to export rather than hand you that file, and it names the edge and how many millimetres short you are. That check is why a simple "is the bit smaller than the pocket" test is not enough.
Picking a style for real work
For plywood boxes and drawer dividers, start with minimal dogbones and whatever bit is already in the collet. Switch to a T-bone only when an edge fails the length check and the slot has a clear long axis. Switch to a full corner when the joint is hidden and you want maximum clearance, or when the edge is too short for anything else.
This version treats your polygon as pocket geometry — the material being removed. Convex 90° vertices get relief; a reflex 270° vertex on an L-shaped pocket is left alone, because the bit already has room there. Outline and outside-cut modes exist in the geometry kernel but are deliberately not exposed, since mixing them with pocket mode is how people mill the wrong side of a line.
Download SVG for LightBurn and Inkscape, DXF for Carbide Create, then run a normal inside or pocket toolpath with the same bit. The exported contour is the finished pocket, so do not offset it by the cutter radius a second time.