A wall is rarely one thing. A shopfront has an arcade below and windows above, with a moulded band between them; the bays below are wide and the ones above are narrow, and none of it lines up. Build that as a single 4 m wall and you have made one building. Split it properly and you have made a town.

The rest of a building has its own pages: Roofs for pitch and the gable, Openings for what
fills a hole, Balconies for what projects out of one. All three are the four roles below applied to a different surface.

This page is about how to split, and it is not only about walls. A roof is slope panels, dormer inserts, ridge and gutter trim, and hip corners. A fence is panels, a gate, a cap rail and a corner post. A pipe run is straights, valves, flanges and elbows. The same four parts, every time.

Every part of a split element is one of four things. Decide which before you model it, because the role decides the pivot, and the pivot is the expensive thing to get wrong.

insert, trim and corner are new as of 2026-09-21 (see What changed). Two of the three exist because they cannot reuse wall: bottom-left means bottom-left of the bounding box, and their bounding box is not the thing that has to line up.

MSH009 checks it, within 0.001. The full table with the reasoning for the older classes is on Modular 3D Assets > Pivots. Set the class on each collection in the add-on’s Asset Rules panel. All three are in the dropdown.

The diagram above is this facade: three wide arcade bays below, five narrow window bays above. The two storeys line up on exactly one thing: the height they stack at.

Within a storey, split on bayWidth: 1 m, the same number as grid.primary. Bays are multiples of it, so a 3 m arch bay and a 1 m window bay sit in the same run without anything being nudged.

Before this number existed, wallSegment said a wall is 4 m wide and indivisible, and a facade with two different bay widths could not be authored at all.

The objection to splitting horizontally is that the seam is a long straight line at eye level. Three different things independently say to solve it the same way:

A panel's grid footprint is its wall plane, not its bounding box. Trim, sills and mouldings
project outside the cell and are not counted.

Unstated, this costs one of two mistakes, and both of them look perfectly correct in Blender:

They are the same mistake seen from either end. MSH015 is what prevents both.

Trim is exempt from MSH015, because projecting is its whole job. MSH016 is what holds it to a limit instead:

grid.trimProjectionMax is 0.25 m, and grid.trimProjection (0.1 m) is the usual. That number decides whether a cornice collides with the piece beside it, or with the one on the return of a corner, and nobody guesses it the same way twice.

The add-on measures it. A trim piece pivots at the wall plane (its +Y face), so everything it has lies forward of that, and the projection is simply its depth. Set a collection's class to Trim and anything past 0.25 m is reported. There is intentionally no minimum. A trim 1 mm proud will not read from the street, but that is a judgement about the building rather than a fact about the geometry.

The grammar needs no change. The three Descriptor slots already allow all of
this. One convention:

A Kit/<Set> folder of sixty pieces then sorts into its four roles alphabetically, for free. No new token, no new folder level, no change to anything that already parses.

Every part is still Kit, still carries the Set and SetNum, still has no SubCategory. Splitting an element does not make the parts a different category.
→ Modular 3D Assets > Kit Piece Naming

Do not hand a level designer forty parts. Splitting a wall into six multiplies their placement work by six, and they will be right to resent the kit.

Ship the parts and a handful of assembled facades as prefabs:

They place a facade and open it up only when they want variation. PRF001 exempts an assembling prefab from mirroring any one model's name. It gets its own Descriptor, exactly like this.

This step is what makes splitting a net gain downstream rather than a tax. Skip it and the kit is technically more flexible and practically slower to use.

→ Build a Level

Half the cost of splitting is five people naming one thing five ways. Cornice / Ledge /
Moulding / Band are one shape at three heights; Pier / Pilaster / Column are three shapes doing one job.

Moulding and Molding both report under LEX002. A moulding is the profile, not the piece, and which one it is decides where on the wall it goes. Storey is British English and never Story, which also keeps it clear of narrative content.

Band, Ledge and Trim are intentionally not in the replaced list: a metal band round a barrel and a rock ledge are both correct, and the rule cannot tell them apart. Those stay a judgement.
→ The Words We Use

2026-09-21: insert, trim and corner pivot classes; MSH015 grid footprint and MSH016 trim projection; bayWidth, storeyHeight, groundStoreyHeight, trimProjection, trimProjectionMax and windowOpening in the grid; a Wall parts vocabulary group.

2026-10-06: windowOpening replaced by three windowOpenings (standard, wide, high), doorOpening raised to 2.25 m so window heads line up with it, and stairRise split into stairRiser and stairGoing. → Openings

Later the same day, MSH016 became enforced in Blender rather than only written down, and MSH018 checks the word a piece is named for against the class it declares.
→ Modular 3D Assets > Which one is it?

All of it lands as warning, like every new rule.
→ Check Your Work > When a rule is wrong

The Blender add-on has the three classes in its Asset Rules dropdown, places the trim and corner pivots, and offers a starter box for both. A corner is 0.2 × 0.2 × 3 m, trim is 4 × 0.1 × 0.2 m.
insert has no starter box, for the same reason door has never had one: it is cut into a wall rather than being one.

Trim gets the flat-connecting-face check too, and it is the one class whose tiling direction is measured rather than looked up: a cornice runs along X and a quoin run along Z, so the add-on reads it off the piece’s own longest dimension.