Learned from Sandi Metz, and the specific arrangement of letters from a former coworker;

HECL (pronounced like heckle) - A better version of SOLID

Highly Cohesive - An object's lifecycle, responsibilities, and API should all fit well together and make practical sense

Easily Composable - A complex organization of objects should be easily made from smaller, independent objects

Context Independent - An object should not be overly specialized and capable of being used in multiple contexts

Loosely Coupled - When an object depends on another, it should "know" as little as possible about the dependency, and ideally the dependency should be injected as an initializing attribute