Internal and External Intercostal Muscles

Internal comes from the Latin word internus, which means within.
External means outside of.
The word intercostal comes from the Latin words inter, which means between, and costa, which means rib.
The internal intercostals then, are muscles between the ribs and located inside a structure, while the external intercostals are muscles between the ribs and located on the outside of a structure.

In the case of the intercostals, they are internal or external relative to one another.

Where do the internal and external intercostal muscles attach?

Origin

The internal intercostals originate and insert between adjacent ribs.
The external intercostals originate on the inferior border of the rib above.

Insertion

The external intercostals insert on the superior border of the rib below.

What actions do the internal and external intercostal muscles do?

The internal intercostals bring the ribs down during forced expiration.
The external intercostals elevate the ribs in inspiration.
Keep in mind that the abdominal muscles all have a role to play in forced exhalation.

Poses where we may be more aware of the internal and external intercostal muscles and breathing in yoga:

In postures like marichyasana C, we may notice that it is more difficult to breathe. The abdomen is compressed during the twist, so the ribs need to move more during the act of inhalation to accommodate that. The ribcage itself is also under more pressure from being stretched. This makes it harder for the ribs to separate and allow air in.

A posture such as parighasana lengthens each side of the body. The ribs are being pulled apart from one another and this would lengthen the intercostals.