Case

Which case (lowercase, camelCase, …) to use and when

Agenzia per l'Italia Digital & Team per la trasformazione digital (Italian administration) - Interoperability model for the Public Administration

The properties of JSON objects must have a consistent naming
snake_case camelCase

Cisco - API Design Guide

3.3 Representations
URL Paths

Cloud Foundry - Cloud Controller API v3 Style Guide (Proposal)

Query Parameters
Field Names

Google - Google API Design Guide

Collection ID
Must be in plural form with lowerCamel case
Camel Case

Heroku - HTTP API Design Guide

Downcase paths and attributes

Zalando - RESTFul API Guidelines

Property names must be snake_case

adidas - adidas API design guidelines

Naming Conventions
JSON