What Are Model Space and Paper Space?

In model space, a 12 foot wide room is 144 units wide in an architectural drawing set to inches. Drafters draw with real dimensions and snap to real geometry, and they never think about scale while placing walls, ducts, or equipment.

Model space also hosts the xrefs: architectural backgrounds, structural grids, and survey linework all overlay at the same coordinates. A reference drawn in feet against one drawn in inches will be off by a factor of 12, which is easy to spot and still common with exported files.

A layout represents the sheet at plot size. On a 36 by 24 inch ARCH D layout, the title block is inserted at 1:1 and a note meant to plot 1/8 inch tall is drawn 1/8 inch tall. Viewports are cut into that sheet to show model space at scale.

Each layout carries its own page setup: plotter, paper size, plot style table, and plot area. Storing named page setups in the firm template and importing them is how a 50 sheet set plots consistently.

A viewport at 1/8 inch equals 1 foot has a zoom factor of 1/96XP. Once the scale is set, the viewport should be display locked, because a turn of the mouse wheel inside an unlocked viewport changes the scale and the sheet still plots, at the wrong scale.

Layer visibility can differ by viewport through VP Freeze and viewport overrides, so the same model can show furniture in one viewport and hide it in another, or screen the architectural background under an electrical layout.

Notes, tags, and dimensions in model space must be scaled to read correctly through the viewport. Annotative objects handle that by carrying a representation for each annotation scale. Annotation in paper space is drawn at plot size and needs no scaling, but it stays put when the model changes or the viewport is panned.

Firm standards usually say which goes where. A common split keeps tags, room names, and dimensions tied to geometry in model space, and puts sheet notes, keynote legends, and the title block in paper space. Paper space dimensions can be associated with model geometry, though that link breaks easily when xrefs are reloaded or moved.

Dashed lines depend on LTSCALE, PSLTSCALE, and MSLTSCALE. With PSLTSCALE set to 1, line types in a viewport scale with the viewport, so a hidden line looks the same at 1/8 inch and 1/4 inch scale. With PSLTSCALE set to 0, dashes in a small-scale viewport can shrink until the line plots solid.

When dashes look wrong, setting those variables consistently and running REGEN usually fixes the whole sheet. Editing lines one at a time is rarely the answer.