What are the main principles of SOLID you use when designing software systems?

The main SOLID principles applied in software system design:

• S (Single Responsibility Principle) — the principle that a class should have only one reason to change.
• O (Open/Closed Principle) — the principle that software entities should be open for extension but closed for modification.
• L (Liskov Substitution Principle) — the principle that objects of a subclass should be interchangeable with objects of the superclass without affecting correctness.
• I (Interface Segregation Principle) — the principle that it is better to have several specific interfaces than one general-purpose interface.
• D (Dependency Inversion Principle) — the principle that high-level modules should not depend on low-level modules; both should depend on abstractions.

Applying these principles helps create flexible, extendable, and maintainable systems.