# Environment Variables, Domains & Production Config
Environment Variables: The Three Scopes
Vercel has three environment scopes, and understanding them prevents the most common production bugs:
| Scope | When it applies | Example use |
|-------|----------------|-------------|
| Development | vercel dev locally | Local database URL |
| Preview | PR preview deployments | Staging API keys |
| Production | Your live site (main branch) | Production API keys, real Stripe keys |
Setting environment variables via CLI:
`bash
# Add a variable for all environments
vercel env add DATABASE_URL
# Add for production only vercel env add STRIPE_SECRET_KEY --environment production
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What you'll learn:
- Configure environment variables for development, preview, and production environments separately
- Set up a custom domain with DNS and understand Vercel's automatic SSL
- Use vercel.json for redirects, headers, and rewrites