Thread on composition advice! Specifically, triangles and how you can use them. Triangles are a great way to keep the eye moving inside the picture frame, and the uneven number of points keeps the composition dynamic and interesting. 
Four points arranged inside of a square would feel much more static in comparison but could be used depending on the effect desired. The Casas painting above has a feeling of unease, while this Frank Craig painting with a rectangular compostion feels more stable and decorative.
Despite this Bolidni and the following Sargent paintings being very busy, there is an organization to them made up of both a central triangle and a supporting scaffolding of parallel angles.
These major angles keep the composition coherent and support the high frequency detail on top, much like how a simple rhythm section keeps structure during a guitar solo. The triangle even serves a narrative purpose, suggesting a relationship between the three figures involved.
This painting uses a scaffolding of limited parallel angles, but the points of the deer and the figure are focal points not because of value contrast, but narrative and psychological contrast. We instinctively look for figures, faces, and story. Contrast comes in many forms.
This kind of abstract design is independent of subject matter and exists in all visual art regardless of genre. It is a staple of sci-fi design, like this design by John Liberto. Design exists in the macro as well as the micro sense. I hope this has been helpful, more to come!
Composition is best used in practical ways to support the intention of the painting and is unfortunately often mystified and complicated to the point where it is too esoteric for beginners to become interested in.
