<!-- SPDX-License-Identifier: MIT -->
<!-- SPDX-FileCopyrightText: 2025-2026 Marcus Quinn -->

# Design System: Apple — Typography Rules

## Font Family

- **Display**: `SF Pro Display`, with fallbacks: `SF Pro Icons, Helvetica Neue, Helvetica, Arial, sans-serif`
- **Body**: `SF Pro Text`, with fallbacks: `SF Pro Icons, Helvetica Neue, Helvetica, Arial, sans-serif`
- SF Pro Display is used at 20px and above; SF Pro Text is optimized for 19px and below.

## Hierarchy

| Role | Font | Size | Weight | Line Height | Letter Spacing | Notes |
|------|------|------|--------|-------------|----------------|-------|
| Display Hero | SF Pro Display | 56px (3.50rem) | 600 | 1.07 (tight) | -0.28px | Product launch headlines, maximum impact |
| Section Heading | SF Pro Display | 40px (2.50rem) | 600 | 1.10 (tight) | normal | Feature section titles |
| Tile Heading | SF Pro Display | 28px (1.75rem) | 400 | 1.14 (tight) | 0.196px | Product tile headlines |
| Card Title | SF Pro Display | 21px (1.31rem) | 700 | 1.19 (tight) | 0.231px | Bold card headings |
| Sub-heading | SF Pro Display | 21px (1.31rem) | 400 | 1.19 (tight) | 0.231px | Regular card headings |
| Nav Heading | SF Pro Text | 34px (2.13rem) | 600 | 1.47 | -0.374px | Large navigation headings |
| Sub-nav | SF Pro Text | 24px (1.50rem) | 300 | 1.50 | normal | Light sub-navigation text |
| Body | SF Pro Text | 17px (1.06rem) | 400 | 1.47 | -0.374px | Standard reading text |
| Body Emphasis | SF Pro Text | 17px (1.06rem) | 600 | 1.24 (tight) | -0.374px | Emphasized body text, labels |
| Button Large | SF Pro Text | 18px (1.13rem) | 300 | 1.00 (tight) | normal | Large button text, light weight |
| Button | SF Pro Text | 17px (1.06rem) | 400 | 2.41 (relaxed) | normal | Standard button text |
| Link | SF Pro Text | 14px (0.88rem) | 400 | 1.43 | -0.224px | Body links, "Learn more" |
| Caption | SF Pro Text | 14px (0.88rem) | 400 | 1.29 (tight) | -0.224px | Secondary text, descriptions |
| Caption Bold | SF Pro Text | 14px (0.88rem) | 600 | 1.29 (tight) | -0.224px | Emphasized captions |
| Micro | SF Pro Text | 12px (0.75rem) | 400 | 1.33 | -0.12px | Fine print, footnotes |
| Micro Bold | SF Pro Text | 12px (0.75rem) | 600 | 1.33 | -0.12px | Bold fine print |
| Nano | SF Pro Text | 10px (0.63rem) | 400 | 1.47 | -0.08px | Legal text, smallest size |

## Principles

- **Optical sizing as philosophy**: SF Pro automatically switches between Display and Text optical sizes. Display versions have wider letter spacing and thinner strokes optimized for large sizes; Text versions are tighter and sturdier for small sizes. This means the font literally changes its DNA based on context.
- **Weight restraint**: The scale spans 300 (light) to 700 (bold) but most text lives at 400 (regular) and 600 (semibold). Weight 300 appears only on large decorative text. Weight 700 is rare, used only for bold card titles.
- **Negative tracking at all sizes**: Unlike most systems that only track headlines, Apple applies subtle negative letter-spacing even at body sizes (-0.374px at 17px, -0.224px at 14px, -0.12px at 12px). This creates universally tight, efficient text.
- **Extreme line-height range**: Headlines compress to 1.07 while body text opens to 1.47, and some button contexts stretch to 2.41. This dramatic range creates clear visual hierarchy through rhythm alone.
