In the University of Minnesota’s scanned set of the Pillsbury Mansion drawings there is a sheet catalogued as side elevation. It is not a side elevation. Open it and the draughtsman’s own lettering across the bottom reads LONGITUDINAL SECTION — SCALE 1/4″ = 1′-0″, and the title block in the corner reads Sheet No. 11.

That makes it the companion to the drawing we read a few days ago: Sheet No. 12, the transverse section, which cuts the house across its width. Sheet No. 11 cuts it the long way. It is the second filename in this set that turns out to describe the wrong drawing — and the second time the sheet underneath has been more interesting than its label.

What a longitudinal section actually shows

An elevation is the outside of a wall. A section is what you would see if you sliced the building open and walked round to look at the cut face. Sheet No. 11 slices the mansion lengthwise, and the result is the whole house stacked up in one drawing: the basement with its arched openings at the bottom, then three floors of rooms, then the roof, with four chimney stacks running the full height and a balustraded deck drawn between two of them.

No furniture. No panelling. No oak. Just the bones — which is exactly why it is worth reading. Every other surviving image of this house shows you the English interiors. This one shows you what is behind them.

The answer, lettered on the drawing, is steel

The draughtsman annotated the structural members by hand, and at full scan resolution the notes are legible. Three of them:

• “15″ Channel 33#” at the roof line — a rolled steel channel fifteen inches deep, weighing thirty-three pounds to the foot.
• “8″ 11.25# Channel” along the pitch of the roof.
• “12″ Channel 15” running up the rake of a staircase, with small 2 × 2″ angles lettered against the individual treads.

Read that last one again. A staircase in this house is not carried on a timber stringer. It is carried on a twelve-inch steel channel, with steel angles bracketed off it to take each step. Whatever went on top of it — and in this house what went on top of things was usually four-hundred-year-old English oak — the thing doing the actual work is a rolled section that came out of an American mill.

We have written before about why the mansion was built like a skyscraper. Sheet No. 11 is where that becomes something you can point at rather than something you have to take on trust.

Two more words worth noticing

Elsewhere on the sheet the draughtsman lettered “Suspended Ceiling” at basement level, and “Granite Sills” at both ends where the wall meets the ground. A dimension of 14′-6″ is pencilled beside the stair.

A suspended ceiling in 1910s practice was not the office-tile thing the phrase now suggests; it means a ceiling hung below the structure rather than fixed straight to it — the standard way to hide beams, ducts and pipes, and a hint at how much servicing this house needed. The granite sills are the point where the expensive stonework of the exterior actually stops.

What the drawing does not say

Two honest limits. First, the sheet does not name which staircase it has drawn. A house this size had more than one, and a longitudinal cut catches whatever falls on its line. We are not going to tell you this is the celebrated staircase from Rotherwas Court — the drawing simply does not say so, and that staircase has a story of its own that does not need help.

Second, the title block on Sheet No. 11 is blank. Drawn by, Checked, Accepted, Order No., Date, Revised — every line is empty. Sheet No. 7, the front elevation, carries revision dates in a working hand. This one carries none, which is a reminder that a set of drawings is a working document and not a finished record. A drawing states an intention. Only the building states an outcome.

The address lettered in the title block is North East Cor. 1st Ave S. & 22nd St., which is not the address the house answers to today. We have left it as the draughtsman wrote it. The firm block reads Hewitt & Brown, Architects~Engineers, 716 4th Ave. South — the same practice named on every other sheet, and the subject of our piece on who actually built the mansion.

Come and stand in it

Drawings are one way to understand a building. Standing under the ceiling that a fifteen-inch channel is holding up is another. The mansion at 100 East 22nd Street is open for guided tours year round, and the experiences and tours page has what is running and when; plan your visit covers hours and how to book.

If you want the rest of the drawing set, start with the side and rear elevations and work inwards. And for the wider story of how Minneapolis learned to build in steel at all, the Mill City Museum is the other half of the same century. The mansion’s own listing sits on the National Register of Historic Places, and the National Park Service’s Preservation Briefs are the standard reading on why old drawings like this matter to anyone repairing a historic building. Historic Minneapolis newspapers, if you want to go looking yourself, are indexed at the Minnesota Historical Society.

Keep reading: Whose Coats of Arms Are Carved Into the Pillsbury Mansion?