Phase

An object’s phase is one of four states it can be in, and describes which other objects that object can interact with.

In-Phase
In-Phase is the most common state, and is the default. If not otherwise specified, an object is In-Phase. Almost everything found in Qud is in this phase state.

In-Phase objects can interact with other In-Phase objects (as well as Omniphase objects), but not with Out-of-Phase objects.

In-Phase is the lowest-priority phase state.

Out-of-Phase
Out-of-Phase is the second-most common state, indicated by the Phased effect. It is normally only achieved through special means, such as the Phasing mutation, or a.

Out-of-Phase objects can interact with other Out-of-Phase objects (and Omniphase objects), but cannot interact with In-Phase objects.

Because almost everything in Qud is In-Phase, the ability to switch to Out-of-Phase can be very powerful, allowing one to walk through walls or escape from attacks by In-Phase enemies, among other things.

Out-of-Phase is the second-lowest priority phase state.

Omniphase
Omniphase is the second-rarest state, indicated by the Omniphase effect. It is restricted to only a few objects, such as the beam of a or the shots of a.

Omniphase objects can interact with both In-Phase and Out-of-Phase objects, as well as other Omniphase objects.

Omniphase is the second-highest priority phase state.

Nullphase
Nullphase is the rarest state, with the only nullphase objects being those which intrinsically have the  tag.

Nullphase objects are unique in that they cannot interact with any other objects, including other Nullphase objects.

Nullphase is the highest-priority phase state.

Achievements
Switching from Out-of-Phase to In-Phase while inside an In-Phase wall will either place the player in adjacent empty tile or, if there are no adjacent empty tiles, will instantly kill the player and grant the The Laws of Physics Are Mere Suggestions, Vol. 1 achievement.