Icon System

An icon set without shared rules decays one icon at a time: each new glyph is drawn against a slightly different grid, stroke, or metaphor, and within a year the product carries three visual dialects that no single redraw can fix cheaply. This skill produces the spec that makes every icon - including ones drawn by future contributors - feel like one family, and the audit checklist that catches drift before it ships.

Operating procedure

Follow the order: the grid constrains the stroke, the stroke constrains the drawing rules, and naming/export only matter once the drawing rules are fixed. Changing the grid after 200 icons exist means redrawing 200 icons.

Step 1: Gather inputs

Collect before drawing anything. Label assumptions as guesses.

1. Rendering sizes in product (commonly 16, 20, 24px). The smallest real size drives detail decisions.
2. Style direction: outline, filled, or both. If both, they are two coordinated sets, not one.
3. Existing icons to keep - an audit target - or a greenfield set.
4. Platform constraints (web currentColor inheritance, iOS/Android asset pipelines).
5. Who will draw future icons - the spec must be executable by someone who was not in the room.

Step 2: Define the grid and keylines

Pick a base canvas, standard is 24x24px. Inside it define:

• Live area - 20x20, where the icon body sits.
• Padding - the 2px margin on all sides; nothing crosses it except deliberate optical overshoot.
• Keylines - reference shapes that anchor optical size: a 20x20 square, a 22px-diameter circle, a 22x16 horizontal rectangle, and a 16x22 vertical rectangle. A circle drawn to the square keyline looks smaller than the square; the oversized circle keyline compensates.

Step 3: Fix the stroke and corner language

• One stroke weight for the whole set: 2px at 24px is the common default; 1.5px reads lighter and more editorial. Never mix weights - mixed weights are the single fastest way to break cohesion.
• Standardize caps and joins (round vs butt caps) and one corner radius for stroke endpoints.
• Decide interior corner radius (commonly 0.5-1px at 24px) and apply it everywhere.

Step 4: Set drawing rules

• Align edges to the pixel grid at the base size; off-pixel edges render blurry.
• Optical, not mathematical, sizing: circles slightly overshoot the live area to match squares; triangles and diagonals are optically centered, not geometrically centered. Test glyphs side by side at target size and nudge until none looks heavier.
• One canonical form per recurring metaphor - a "plus" is always the same plus, reused, never redrawn.
• Pick one detail level and hold it; a detailed glyph next to a minimal one reads as two sets.
• If filled variants exist, define the outline→filled transformation rule once (which strokes become fills, which counters stay open).

Step 5: Name by meaning

Names describe function, not appearance, so a visual redesign never forces renames:

• delete not trash-can; settings not gear.
• Pattern: name-variant with optional category prefix - arrow-left, arrow-left-filled.
• Lowercase, hyphenated, singular.
• Maintain an alias table mapping synonyms to canonical names (remove → delete, preferences → settings) so searches succeed.

Step 6: Define the export pipeline

• Export SVGs with a uniform viewBox="0 0 24 24" and no width/height attributes.
• Use fill="currentColor" (or stroke) wherever color should inherit; no hard-coded brand hex values in the file.
• Strip editor metadata and run every file through an SVG optimizer (SVGO or equivalent); a clean icon is typically under 1KB.
• Flatten strokes to outlines only if the platform requires it - outlined strokes cannot be re-weighted later, so keep stroke sources.

Step 7: Redraw for small sizes

Never scale 24px art down to 16px - it goes muddy. For 16px, redraw with reduced detail, simplified counters, and a stroke that stays near 1.5-2px rendered. Ship size variants as separate optically-tuned files (icon-name-16, icon-name-24).

Worked icon spec (copy and fill)

ICON SYSTEM SPEC - [FILL: product name]

GRID
  Canvas: 24x24px    Live area: 20x20    Padding: 2px
  Keylines: square 20x20, circle Ø22, h-rect 22x16, v-rect 16x22

STROKE
  Weight: 2px (never mixed)   Caps: round   Joins: round
  Corner radius: 1px interior, round endpoints

DRAWING RULES
  Pixel-aligned edges at 24px. Optical centering for diagonals.
  Circles overshoot live area to Ø22. One detail level: [FILL: minimal / moderate]
  Canonical metaphors: plus, chevron, arrow drawn once, reused everywhere.

NAMING
  Pattern: name-variant (lowercase-hyphenated-singular)
  Examples: arrow-left, arrow-left-filled, delete, settings
  Aliases: remove→delete, preferences→settings, [FILL: more]

EXPORT
  viewBox="0 0 24 24", no width/height, fill="currentColor"
  SVGO-optimized, no editor metadata, <1KB target
  16px variants redrawn, suffixed -16

SIZES SHIPPED: [FILL: 16 / 20 / 24]
OWNER OF FUTURE ICONS: [FILL]

Consistency audit checklist

Run against an existing set; every "no" is a defect to log:

• Same canvas and live area on every file? Same stroke weight measured, not assumed?
• Caps, joins, and corner radii uniform? Detail level uniform?
• Circles/squares/rectangles optically equal at target size when placed side by side?
• Edges pixel-aligned at base size? Any glyph blurry at 100% zoom?
• Names follow the pattern, meaning-based, no orphan synonyms outside the alias table?
• SVGs share one viewBox, inherit color, pass the optimizer with no diff?
• Small sizes redrawn rather than scaled?

Deliverable

Produce an icon system spec containing: the grid and keyline definitions, stroke and corner rules, drawing rules with optical-sizing notes, the naming convention plus alias table, the export pipeline checklist, and - for audits - the defect list from the checklist with a fix priority (grid violations first, naming last, because grid fixes require redraws while renames are mechanical).

Do NOT

• Do not mix stroke weights or detail levels - either one instantly reads as two icon sets.
• Do not center diagonal forms geometrically - they will look off-balance; center optically.
• Do not scale 24px icons to 16px - redraw them; scaled icons render muddy.
• Do not name icons after their appearance - gear breaks the day the settings icon becomes sliders.
• Do not hard-code fills in exported SVGs where the product needs color inheritance - every theme change becomes a re-export.
• Do not change the grid after the set is large without budgeting a full redraw; there is no partial migration.

Quality bar

• A designer who never saw the set can draw a conforming new icon from the spec alone, without asking a question.
• All keyline shapes look the same size side by side at the smallest shipped size.
• Every file passes the audit checklist clean.
• Icon color changes in product via CSS alone, with no asset re-export.

Pair with design-handoff-doc when the spec ships to engineering alongside other design deliverables, and with design-qa-checklist to verify implemented icons match the spec post-build.