Paths, Strokes, and Fills
Every diagram element is a path. Every path has a stroke, a fill, or both. Understanding how to control these independently is the foundation of everything else.
Why this matters
In AutoCAD, a line is a line and a hatch is a separate object. In Illustrator, a single path simultaneously carries the edge (stroke) and the interior (fill) as attributes of the same object. This seems simpler — until you need to control them independently across a diagram. When two adjacent diagram areas share an edge, should that edge appear as a stroke? Sometimes yes, sometimes no. Managing this requires understanding strokes and fills as separable systems, not as a single package.
The fundamental logic
Every Illustrator path has two independent attribute channels: stroke (the line along the path) and fill (the color or pattern inside it, for closed paths). Each can be set to None, a solid color, a gradient, a pattern, or various effects — independently of the other.
| Attribute | What it controls | Common values |
|---|---|---|
| Stroke color | The line color along the path | Solid color, None (no visible stroke) |
| Stroke weight | Line thickness in points (1pt = ~0.35mm) | 0.25pt (hairline) to 2–3pt (heavy boundary) |
| Stroke dash pattern | Solid, dashed, or dotted line appearance | Match conventions from CAD — dashed for existing, dotted for proposed, custom for diagram elements |
| Stroke caps | How open path endpoints appear | Round, Butt, or Projecting — matters for dashed line appearance |
| Stroke joins | How corners look at path vertices | Miter (sharp), Round, or Bevel — affects diagram area boundary corners |
| Fill color | Interior color of a closed path | Solid color, gradient, pattern, or None |
| Fill opacity | Transparency of the fill (separate from stroke opacity) | 0–100% — partial opacity allows layering of diagram areas |
Separating stroke from fill — the professional workflow
The default Illustrator path carries both stroke and fill on the same object. This creates a specific problem for diagrams: when two filled areas share a boundary, both paths have strokes — producing a doubled border at the shared edge. More importantly, if you want some boundaries to show and others not to, you cannot control this per-edge with a single-object approach.
The professional solution: use separate objects for fill areas and edge lines.
- Fill objects: closed paths with a solid fill color, stroke set to None
- Edge objects: open or closed paths with a stroke color and weight, fill set to None
This separation means you can turn off all edges independently of all fills, control which boundaries show as strokes without affecting the filled areas, and stack fills with different opacities below a single clean linework layer. It is more work to set up and more objects to manage — and it produces diagrams that are far more controllable.
Stroke weight and visual hierarchy
Illustrator uses points as the stroke weight unit (not millimeters, as in AutoCAD). As a rough reference: 0.25pt is hairline, 0.5–1pt is standard linework, 1.5–2pt is medium emphasis, 3pt+ is heavy boundary. There is no CTB-equivalent in Illustrator — lineweight is set directly on each stroke object. Maintain hierarchy intentionally: site boundary heavier than program areas, program areas heavier than subdivision lines, subdivision lines heavier than notation.
Try this
Draw three adjacent closed shapes representing adjacent program areas. Apply fills with stroke on the same objects — observe the doubled borders. Then delete the strokes from the fill objects and draw separate edge lines over the boundaries. Compare the control you have in each case. Then try hiding just the edge layer — notice how the fills remain while the linework disappears entirely. This is the control that separate objects give you.
What breaks
Fill on an open path — Illustrator will attempt to fill an open path by drawing an imaginary straight line between the two endpoints. The resulting shape is rarely what you intended. Close paths deliberately, or set fills only on confirmed closed paths.
Stroke weight not reviewed at output scale — a 0.5pt stroke looks reasonable on screen at 100% zoom. At the printed or pinned size of 11×17", it may be too light to read. Review your work at the actual output scale before finalizing weights.