Pollock was one of the group of abstract impressionists seeking to find a new means of expression and a response to modernism as the world adjusted to peace after the horrors of World War Two.
From out of this Jackson Pollock developed his 'action' style of painting. This is an almost autobiographical style. It inverts the traditional idea of a painting, where the canvas is a mirror of something external which the artist is trying to communicate through paint.
The purpose of action paintings is to tell the story of the moment when the artist is painting. Instead of communicating something external it is a record of the moment and what happened in that moment and how Pollock felt in that moment.
The canvas has a sense of peace in amongst the frenetic energy of the painting process. The darkness of the black is mostly covered up by the paler colours that Pollock chose to use, making it a juxtaposition of a work- highly energetic and yet peaceful and calming all at once.
Number 27 achieved unexpected infamy in 1999 when Whitney Museum of American Art hung the work vertically rather than horizontally. This was intentional on the part of the museum- it had been hung the same way in a 1950 exhibition at The Betty Parsons Gallery.
More than anything this points to the ambiguity that Pollock's painting process has on his work. Pollock himself did not work from one side of a canvas to another in a linear style, instead preferring to travel around the canvas.
As such the canvas is neither horizontal nor vertical, it just is, something that The Whitney perfectly illustrated.