The main Rails directories include:

• app/ - Core application code (models, views, controllers, helpers, assets)
• config/ - Application configuration, routes, database settings
• db/ - Database schema, migrations, seeds
• lib/ - Extended modules and custom libraries
• public/ - Static files directly served by web server
• test/ or spec/ - Test files
• vendor/ - Third-party code
• Gemfile - Gem dependencies specification