Drafting Georgian fanlight tracery is an exercise in applied classical geometry. Radiating outward from a central semi-circular hub or carved patera, radial rays divide the semi-circle into odd-numbered symmetrical sectors (typically 7, 9, 11, or 13 bays). Secondary intersecting Gothic lancets, teardrop loops, and batwing arcs are scribed using beam compasses to create dynamic visual motion.
At a Glance: Key Takeaways
- Odd-Number Radial Symmetry: 7, 9, 11, or 13 divisions ensuring a single vertical ray at the top crown apex.
- Teardrop & Batwing Tracery: Intersecting circular arcs generating elegant looping petal profiles.
- True Semicircle vs Trammel Ellipse: Geometric methods for drafting segmental and elliptical entrance arches.
- Focal Hub Proportions: Central hub radius calibrated to precisely 1/4 to 1/6 of the total fanlight radius.
- Optical Parallax Compensation: Outer spandrel rays spaced slightly wider to counteract foreshortening.
1. Mathematical Division of the Semicircular Arc
A fundamental rule of Georgian neoclassical proportion is that a fanlight must always have an odd number of radial sectors (most commonly 7, 9, 11, or 13 bays). An odd number guarantees that a single radial bar sits dead-vertical at the 12 o'clock crown apex, reinforcing the structural keystone axis of the masonry arch above.
Given a total semicircular arch span \(2R\), the master joiner drafts the semi-perimeter arc length \(S = \pi R\). To divide this arc into \(N\) equal sectors using a straightedge and compass:
For a standard 9-ray sunburst, \(\theta = 20^\circ\). Scribing lines from the focal center point through these division points establishes the primary structural spoke axes of the window.
2. Geometry of the Teardrop Loop & Intersecting Lancets
The iconic teardrop (or peacock tail) tracery popularized by Robert Adam is constructed through intersecting concentric arcs. Along each primary radial spoke, the joiner establishes two intermediate radius circles: \(R_1 = 0.45 R\) and \(R_2 = 0.75 R\).
Using the intersection points on \(R_1\) as compass centers, arcs are swung with radius \(r = R_2 - R_1\). These arcs sweep outward from adjacent spokes to meet in an inverted Gothic pointed cusp along the intermediate axis. The result is a ring of undulating teardrop petals that visually soften the rigid mathematical rays into a flowing floral dance.
3. The Trammel Method for Elliptical Doorways
While early Georgian doors were true semicircles (aspect ratio 2:1), late 18th-century townhouses featured wide, low semi-elliptical transoms. An ellipse cannot be drawn with a single compass center; it has semi-major axis \(a\) (half width) and semi-minor axis \(b\) (rise height).
Georgian joiners drafted perfect ellipses using the Trammel of Archimedes or the string-and-pin method based on focal points \(F_1\) and \(F_2\):
To divide an ellipse into radial rays that appear visually uniform to the human eye, architects did not use simple equal angles (which causes bunched rays at the springline and spread rays at the crown). Instead, they projected points from an auxiliary circumscribed semicircle downward onto the ellipse, ensuring harmonious optical rhythm across the entire curve.
Radial Division Proportions by Fanlight Width
Use this reference table to calculate exact physical dimensions, ratios, and specifications before purchasing:
| Doorway Clear Width | Arch Geometry | Optimal Sector Count (N) | Focal Hub Radius |
|---|---|---|---|
| 900mm to 1,050mm (Narrow) | True Semicircle | 7 sectors (θ = 25.7°) | 85mm (1/6 span) |
| 1,100mm to 1,350mm (Standard) | True Semicircle or Segmental | 9 sectors (θ = 20.0°) | 110mm (1/6 span) |
| 1,400mm to 1,800mm (Monumental) | Semi-Ellipse (a=750mm, b=450mm) | 11 to 13 sectors (projected) | 140mm (1/5 span) |
Geometry & Glazing Constraints
Every successful decoration project must account for physical and environmental constraints:
Acute Angle Glass Cutting
Where radial rays meet the central hub, angles can drop below 15°; glass panes must be cut with a 1.5mm relief radius at sharp points to prevent cracking.
Masonry Arch Settlement
Historic brick arches frequently settle into slight asymmetries; always scribe physical templates directly from the in-situ stone doorcase.
Drafting Protocol for a 9-Ray Teardrop Sunburst Fanlight
Execute this professional sequence to achieve flawless results without backtracking:
Springline & Center Hub Layout
Draft horizontal springline baseline and vertical centerline; strike the central hub semicircle with radius R/6.
Outer Arch Scribing
Strike the finished outer arch contour using beam compass or elliptical trammel from the established center.
180° Arc Angular Division
Divide the outer semicircle into 9 equal 20-degree sectors using protractor or geometric chord steps.
Radial Spoke Ray Drafting
Draw straight radial lines from the hub perimeter to each division mark on the outer arch.
Concentric Intermediate Circle Strikes
Strike two intermediate concentric arcs at 45% and 75% of the total radius to establish teardrop cusp baselines.
Teardrop Intersecting Arc Scribing
Set compass to inter-spoke chord width; strike sweeping arcs from adjacent spokes to form the looped teardrop cusps.
Common Mistakes & Countermeasures
Avoid these costly errors that compromise aesthetics, ergonomics, and budget:
Drafting fanlights with an even number of sectors (e.g., 8 or 10 bays)
Using circular arcs to fake an elliptical arch
Project Execution Checklist
Track your physical progress. Checkboxes persist automatically on this device:
Frequently Asked Questions
What is the difference between a fanlight and a transom window?
A transom window is any horizontal glazed window installed above a door. A fanlight is a specific classical subclass of transom that is semicircular or elliptical, featuring radial glazing bars radiating outward from a central hub like an unfolded hand fan.
Why did Georgian Dublin develop so many unique fanlight patterns?
Dublin's Georgian estates (like the Fitzwilliam and Gardiner estates) enforced strict building leases requiring identical brick facades, sash windows, and rooflines. The only architectural feature where wealthy leaseholders were permitted individual creative expression was the fanlight, sparking a golden age of competitive artisanal design.
