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Every declaration in loft places an element, and almost every placement is written off something already in the model: a grid, a level, another wall. This page covers both halves. First, how to write a reference. Then the methods of placement, one for each kind of element. The examples on this page are one model, read top to bottom. Every block parses, and a test holds the page to that.
Reference, don’t measure. Place elements off grids, levels and other elements, not off project coordinates. A position written as grid.A + 12' moves when grid A moves. A position written as 12' stays where it was typed. Revit keeps that relationship in a locked dimension or an EQ constraint; loft keeps it in the text.

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How to write a reference

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Types

Declare the types first. The elements below take their type parameters from them.
The word right after the keyword is the type: wall EXT-1 W1, door 3070 D1. It stays bare because the keyword already says which kind of type it is; a wall’s type can only be a walltype. wall walltype.EXT-1 W1 also works.

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Elements

Everything else is written category first: grid.A, level.L1, wall.W1. To reach one of an element’s datums, add it after a second dot: wall.W1.end, door.D1.jamb.far. Read it left to right as category, id, datum. The dot only separates.
  • Ids have no dots. A.5 would read as a reference. Name the half-grid A5 and give it label "A.5" for the bubble.
  • Ids are case-sensitive. wall.w1 is not wall.W1.
  • Keep ids unique across the model. A wall.W1 and a window.W1 should not both exist.
  • Rooms, room separation lines and views can’t be referenced. They read the model; nothing is placed off them. room.210 in a slot is an error that says so.
Some slots want a particular element rather than a shape. in wall.W1 names a door’s host, base level.L1 a wall’s base, of level.L1 a plan view’s level.

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Datums: planes and plumb lines

A datum is a plane or a line that other elements are placed from. Loft has two today:
  • A plane, vertical or horizontal. Grids are vertical planes; levels are horizontal ones.
  • A plumb line, a vertical line. In plan it reads as a point, which is why every (x, y) pair in loft is a plumb line.
Points in 3D space and sloped lines are coming. Every datum comes from one of three places. Declared. Grids and levels are declared as datums: grid.A, level.L1. Projected. Every element gives off datums of its own, the way a Revit family exposes its references. A wall projects planes (centerline, core.centerline, foc, fof, top, base) and a plumb line at each end (start, end). An opening projects jamb, head, sill and centerline, and a door adds hinge. Each category page lists its datums. Calculated. Arithmetic on datums makes a new one: grid.A + 12', (grid.B - grid.A)/2, wall.W2.centerline + 1'. Two vertical planes that cross make a plumb line: (grid.A, grid.1), (grid.A - 12', grid.1). Project coordinates such as (4'-6", 1'-2") are the fallback: plain lengths measured from the project origin. Use them when nothing in the model is there to measure from. Planes are unbounded. A wall’s centerline runs on past the wall’s ends, so it can cut something the wall never touches.

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What each slot takes

A slot asks for a shape, not a category. Where it wants a plane, any vertical plane will do, a grid’s or a wall’s. A slot asks for a category only when it names the element to host on or measure within. Two rules hold in every slot.
  • A bare wall is shorthand, and the slot decides the plane. After left of, right of, past or short of it is the core face on your side. As an extent it is the centerline. In a coordinate it is refused; name the plane.
  • A bare number is only ever a factor. The 2 in /2 is arithmetic. Anywhere else a bare 1 is neither the grid nor a length, and the parser asks which you meant: grid.1 or 1'.

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How to place elements

Each element is fixed in place by one geometric thing: a plane, a line, a point, a wall, a face or a loop. That thing is its method of placement. Walls and room separation lines share one method; doors and windows share another.

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By a plane — grids and levels

A grid or a level is itself a plane, fixed by one number: at for a grid, elev for a level. A grid is a vertical plane seen edge-on in plan. A level is a horizontal plane seen edge-on in section.
Write each grid off another and the grid system moves as a set: move A and every grid written off A follows. A level’s story height is worked out from the elevations. above names the level above when it isn’t the next one up, and height states the story height outright. Coming: refplane, a reference plane that is neither a grid nor a level.

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By a line — walls and room separation lines

Walls and room separation lines (roomline) are placed the same two ways: between two plumb lines, or along a carrier between two extents. Height is not placement. base, top and height are wall parameters, Revit’s Base and Top Constraint, and they work with either form. See the wall page.

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Between two plumb lines

from (a, b) to (a, b). A wall’s end can fill a whole pair, which is how one wall chains off the last.
A chain can’t loop back on itself. If W4 starts at W1’s end, W1 can’t be written off W4. A diagonal wall is declared and hosts openings like any other. Its centerline can’t be used in an expression yet; only planes square to the grid can.

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Along a carrier, between two extents

from <extent> to <extent> along <carrier> from <extent> to <extent> at <length> left|right of <carrier> The carrier is where the line is drawn: the plane it rides (along), or the plane it sits a set distance off (at … left of, right of). The extents are where it is cut.
Revit. This is Pick Lines with the offset kept. Revit draws the wall and forgets it came from a grid. Here the grid stays in the text.
Plan: wall W5 drawn along grid B, cut at grid 1 and at grid 2 Plan: wall W5 drawn along grid B, cut at grid 1 and at grid 2 The carrier is grid B, so that is where W5 is drawn. It starts where grid 1 crosses the carrier and stops where grid 2 does. Plan: wall W7 four feet to the right of wall W2, with right read off the direction of travel from grid 1 to grid 2 Plan: wall W7 four feet to the right of wall W2, with right read off the direction of travel from grid 1 to grid 2 The carrier is W2’s core face on the side you named, and W7’s location line sits four feet off it. Right and left are read walking from the first extent to the second, so swapping the extents flips the side. along takes a grid today. To run a wall off another wall, use the offset form.

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What cuts a carrier

An extent is a plane that intersects the carrier, or a plumb line whose straight line to the carrier is perpendicular to it.
Plan: wall W3 on a carrier six feet right of grid B, cut at grid 1 and where the plumb line at the end of diagonal wall W6 is square to the carrier Plan: wall W3 on a carrier six feet right of grid B, cut at grid 1 and where the plumb line at the end of diagonal wall W6 is square to the carrier W6 is diagonal and nowhere near the carrier. Only its end matters, and W3 stops square to it. Because planes are unbounded, an extent cuts the carrier whether or not the two elements touch.

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What an extent can’t be

Plan: three things refused as extents on a carrier along grid B — the parallel grid A, a coordinate, and a bare length Plan: three things refused as extents on a carrier along grid B — the parallel grid A, a coordinate, and a bare length Three things look like extents and are refused. A parallel plane never meets the carrier. A coordinate states two numbers, and the carrier already fixed one of them. A length is a distance, not a datum, and a wall has no station along its carrier.
error=D0608
error=D0236
error=D0234
along takes a grid, not a wall:
error=D0611
A wall has two finish faces, and a coordinate has no way to say which. The same face works after right of, because right picks the side.
error=D0327
An opening’s datums are stations along its host wall. Today they mean nothing outside it.
error=D0314
Coming on the same method: beams, sloped columns, railings, curtain walls, ducts, and paths with more than one segment.

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By a point — rooms

A room is placed at a plumb line, and its level makes that a point. Rooms take grids and project coordinates only, never walls: a room marks the space its walls enclose, and moving a wall must never move the room.
error=D0210
Coming: columns, furniture and equipment, placed at a point and standing on a level or floor, like Revit’s level-based families.

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By a wall host — doors, windows and openings

Doors, windows and openings are cut into a host wall (in wall.W1) and placed by a station: a distance along the wall. The station is measured from the wall’s start unless you say otherwise. past measures forward from a datum; short of measures back from one. The station lands on the opening’s center unless you say to jamb, to jamb.far or to centerline.
WIN3 is Revit’s EQ: two windows in place, a third centered between them. Move either window and WIN3 follows. A station is a length along the wall, and a length can be an expression over datums. A station measures from a grid, a wall (its core face), a wall plane such as wall.W5.centerline, or another opening’s datum in the same wall. It can’t measure to a wall’s end. Use one of that wall’s planes.
error=D0322
It can’t take arithmetic after past or short of. Write the whole station as an expression after at instead.
error=D0213
sill is a parameter, not placement. See the door, window and opening pages.

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By a face host

Coming. Sconces, receptacles, ceiling and floor fixtures: the wall-host method with a face as the host and a point on it as the station.

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By a loop

Coming. Floors, roofs, ceilings and areas are fixed by a closed loop, what Revit calls a sketch.

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Not placed — views, types and annotations

A view is an element that isn’t placed. It is a query over the model and an instance of its viewtype. It says what it looks at: of level.L1 for a plan, a cut plane at and a far clip to for a section. It can also crop to a region in model coordinates. Nothing is placed off a view, and each view lives in its own file.
Types have no position. Annotations, meaning tags, dimensions and text, will sit on a view, never in the model.

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Where to look next

Each category’s datums, and what each of its words takes, are on the category catalog. The one-screen summary of the language is the cheat-sheet.