The Practical Geometry of Boston's Old North Church

first written Feb. 2026, updated Oct.25, 2026 

The Old North Church, in Boston's North End, was built in 1723.*

The church fathers responsible for its construction had visited England where they had probably seen the churches in London built after the Great Fire in 1669. Those churches were Classical, no longer Gothic in design. The Old North Church was built in the fashionable new style.

When I teach/share/explore Practical Geometry with the North Bennet Street School Preservation Carpentry students, we, i the instructors, visit the church and see its construction firsthand: from the crypt with its tombs to the timber roof frame in the attic.    

The school and I have copies of the Historic Structures Report for the church. Written c. 1970. It includes floor plans which trace the evolution of the church interior. These are not HABS drawings, not as detailed and possibly not as accurate. This is, therefore, a preliminary exploration.

The exterior is brick. The floor, balconies, and roof are timber framed. One of the church fathers seems to have overseen the master mason responsible for the foundation and walls. A master timber framer was responsible for the roof. All of these men would have known Practical Geometry. *** 

 

Here is how I think the design for the church was laid out.

The plan shows the church layout in 1730. The entrance and apse are extensions of the main space. The pulpit with its curvy stair is drawn at the left of the apse. In later years the pew locations, the stairs and some exterior doors change. 

The geometry of the exterior walls is simple. It is the 3/4/5 rectangle. The 3/4/5 triangle outlined in red, and lettered A, B, C.

The apse is a semi-circle; its center is at D.

The entry is a square, E.  The walls of the entry are thicker than the church walls because they bear the weight of the steeple.  

The diagonal lines of the Rule of Thirds cross the floor plan. Where they cross are the points - F - which locate the red vertical lines which divide the space in thirds. See the notation across the bottom of the drawing.  

These lines lay out the inside width of the apse and the entry. **** 

The builders wanted us to see the apse, that curve and its half dome when we enter the sanctuary. It's the focus of the church.   

 

The columns support the floor, the balconies and the roof trusses. 

They were not placed to reflect the 3-part rhythm of the entry and apse. 

Those proportions - dividing the width into thirds - would have made the interior of the church feel like a corridor, not a place for gathering.  

Dividing the space into quarters placed the columns and the balconies visually on the edges, allowing more room in the middle. This created a welcoming space in the sanctuary for the congregation.

The drawings show the first Lines which could have laid out the location of the columns. The Lines cross in the center of the sanctuary, dividing it in half. The space is divided into 4 rectangles.

The diagonals of those rectangles cross.

They locate the points necessary to lay out the Lines for the columns. 

 In the next drawing I have added circles at those points.

Their rhythm is 1-2 -1.  The width for the side-aisles and balconies above them is 1/4 of the sanctuary. The center of the church is 1/2 of the width.  

The lines for the columns are beside the columns. They serve as reference points during construction.  The Lines are marked in red.

The Lines also cross just below the first freestanding columns, those closest to the apse. 

The distance between these columns and the pilasters at the apse wall is the distance between all the columns. The space at the rear of the sanctuary is not included as the stairs to the balconies are located there.

The windows:

In a time where candles and torches were the only artificial lighting available, placing the windows for maximum natural light was essential. *****

The arcs in black on the right side locate the center line (C) of the window.  The center of radii for the arcs is at the columns.

How?  Place your compass point on the edge of one column. Swing an arc. Swing a similar arc from the opposite column. The line drawn by the 2 points where the arcs cross will locate the center of a window.

 Notes

*The church is best known for its steeple where Paul Revere hug his lanterns on April 15, 1775, warning the citizens of Massachusetts Bay by which route the British soldiers would march to Lexington and Concord.

 **This drawing is by Suzanne Carlson, from John Fitzhugh Millar's book, The Architects of the American Colonies, Barre Publishers, Barre, MA, 1968. Used with permission from John Fitzhugh Millar.  

*** In 1723 someone who designed buildings was not called an architect. James Gibbs who designed and supervised the construction of St. Martin's in the Fields in London, was chosen by Commissioners to be "the Surveyor of that work" . See Gibbs' Book of Architecture, London, 1728, p. iv. There seems to be no use of a similar word in the church record.

**** Note: In geometry, Practical or otherwise, a Line cannot be drawn without 2 points. To learn about the Rule of Thirds, see  https://www.jgrarchitect.com/2020/08/lesson-6-rule-of-thirds-part-1_21.html

 ***** For a discussion on windows and light see Edward Shaw, Civil Architecture, 6th ed. 1862, 'Windows', beginning on p. 107.