The direct applicability and direct effect of different forms of EU law

Since some forms of EU law are only directly applicable, while others are also directly effective, we obviously need to know which is which. We will consider the treaties, regulations, directives and decisions in turn, but before doing so we will refine the basic question before adding two further ones.

The refinement involves rephrasing the basic question so that it becomes:

Which forms of EU law are capable of being directly effective?

The first additional question is:

Supposing we are dealing with a form of EU law that is capable of being directly effective, how do we know whether this particular provision actually is directly effective?

The second additional question is:

Supposing the particular provision of EU law actually is directly effective, against whom can it be enforced? (If it can be enforced by a private individual or organisation against the state, its direct effect is said to be vertical. If it can be enforced by a private individual or organisation against another private individual or organisation, its direct effect is said to be horizontal.)

To answer this additional question, we need to understand what is meant by the criteria for direct effect.