This page presents the major steps of Piet Mondrian’s oeuvre evolution process from the first naturalistic paintings up to the last Neoplastic abstract compositions.
How does the Dutch painter come to a space based on perpendicular lines and yellow, red and blue colors?
The visible
The artist begins to paint in accordance with the tested canons of the naturalistic painting, otherwise called figurative:
Interpreting the visible
Around 1907, the earlier landscapes of the naturalist phase are stripped of trees, houses, and any sign of human presence and seem to want to express an unspoiled nature:
At the same time the painter’s attention turns to constructions such as mills or lighthouses:
In the compositions that expand horizontally, the gaze is opened to the infinite dimension of nature while in the vertical architectural volumes everything is concentrated in a finite space in which human beings think, design and build to rise from a primitive condition of nature.
While an horizontal extension prevails in the natural landscapes and a vertical development in the architectural volumes, the two opposite directions interpenetrate in the figure of a tree:

Seascape, 1909 with Diagram

Church Tower at Domburg,
1911 with Diagram

Study of Trees 1, 1912 with Diagram

Study of Trees 1, 1912, Black Crayon on Paper, cm. 66 x 89,1 with Diagram
In the tree the branches extend horizontally toward the sides while the trunk leads them back toward the center.
The later abstract compositions formed by horizontal and vertical lines are already present in the figure of the tree although still in a form veiled by appearances.
The visible and the invisible
Around 1912 the space evolves in a cubist sense (Fig. 10 – 11 – 12).
The basic structure of the tree (Fig. 10) reappears within a rectangle at the center of an abstract composition (Fig. 12) where horizontal and vertical strokes now express in a clearer form the disordered multiplicity of branches seen in the naturalistic tree:

Study of Trees 1, 1912 with Diagram

Composition II, 1913 with Diagram
The natural and the spiritual

Composition II, 1913, Oil on Canvas, cm. 88 x 115 with Diagram
The relationship between horizontal and vertical, which is expressed in a rather univocal and static way (Fig. 10), multiplies and takes on ever changing combinations (Fig. 12). One could say that a single tree (Fig. 10) appears now in many different ways albeit in abstract form, that is, in essential terms. While the composition expresses a variety of precarious relationships between opposites, the central rectangle shows a steady relationship between the opposing directions that elsewhere tends instead to become unbalanced in a variety of ever-changing situations The rectangle suggests something more stable and enduring (the spiritual) in a sea of ever changing situations and precarious balances (the natural evolution of existence).
From a rectangle to a square
All this is confirmed by new works that take as their motif a pier extending into the sea (Fig. 13 – 14). The artist probably saw the pier structure as a solid element (like the vertical mills and lighthouses) a symbol of permanence, interpenetrating with the dynamic flow of the sea. Permanence is invoked by the spiritual with respect to the manifold aspect of nature and changeable course of life.
The pier develops from the bottom-center of the composition (like the trunk of the tree) while the sea horizon expands horizontally (like branches of the tree). Same as the trunk with respect to the branches, the vertical pier tends to concentrate the horizontal expansion of nature (the sea):
The interaction between the vertical of the pier and the horizontal of the sea generates a sort of rectangular area (Fig. 14) which then blossoms into a set of vague square proportions with one larger and more defined square in the upper center of Fig. 15. A square form is namely an equivalence of horizontal and vertical and therefore expresses the most balanced of relationships between the opposite directions the composition originates from, that is, the ever-changing, horizontal flow of the sea (nature) and the permanent, vertical form of the pier (symbol of mankind) (Fig. 15):
Just as in the figure of a tree (Fig. 10), the vertical trunk unites a multiplicity of branches, a rectangle (Fig. 12) a square (Fig. 15) unite in a more balanced form a multiplicity of unbalanced relationships between horizontal and vertical strokes. For Mondrian at this stage the square forms conveys a sense of unity and stability (the spiritual) of a mutable multiplicity (the natural):

Composition II 1913 with Diagram

Pier and Ocean 4 1914 with Diagram
An invisible overall design
By reducing the ever-changing appearance of the world to a multitude of orthogonal signs, Mondrian performs an arbitrary operation with respect to our common perception of reality. However, this allows him to express on the contained space of the pictorial surface the widest diversity while maintaining something constant (the perpendicular relationship).
The process of abstraction allows the artist to contemplate the infinite variety of the world without sacrificing the idea of synthesis and unity that arises from his inner world. Each sign is different from the other but they all share the same intimate essence (the orthogonal relationship), just as every single thing in nature is different from the other but they all share the same basic elements that reveal an invisible overall design.
“There is a common design to all things, plants, trees, animals, humans and it is with this design that we should be in consonance.” (Henri Matisse)
The task of faithfully representing the fleeting appearance of things has since been taken over by photography.
Evolution of the square
The square proportion that took shape during the cubist phase (Fig. 15) will inform almost all the works Mondrian paints in the course of the 1920’s and the 1930’s (Fig. 16 – 17 – 18).
The square will be a constant element but one that is in constant flux and unstable balance between opposing directions.
In Fig. 15 we can see a variety of undefined squares around the central one:

Pier and Ocean 4, 1914 with Diagram
The same relationship between a fully accomplished square and some undefined squares can be seen in a following version of the Pier and Ocean theme (Fig. 16):

Pier and Ocean 5, 1915 with Diagram
The squares which remain incomplete evoke a sense of variation (the never fully attained balances between opposites we strive for during our lives) while the central balanced square suggests the possibility to reach the ideal unity invoked by the spiritual within us.
No closed geometric shape
Mondrian does not see the square as a closed and pre-established geometric shape but rather the given moment in which the relationship between horizontal and vertical, that is to say, between opposite drives, reaches a certain balance which is then lost when the different aspects again start to challenge and attain predominance over one another. The balance of the composition is influenced by all the elements and not only by the square.
Just as Mondrian chose out of all the possible relations of form the fundamental one expressing the utmost contrast (horizontal-vertical), in terms of color his eye preferred the fundamental primary colors. Moreover, yellow, red, and blue seemed to him the freshest and the best able to transform the painted surface into a living and exuberant reality.
Throughout the 1920’s the compositions show a variety of square and non-square areas, now white and fully enclosed on four sides (Fig. 19 – 20 – 21), now blue, red or yellow opened on one on two sides (Fig. 20 to 26):
Michel Seuphor describes the canvases of the period 1928-30 as classic Neoplasticism. I agree; some of these works are indeed authentic little masterpieces.
Every Neoplastic composition expresses this dialectic between the changing aspects of life and the human need to stabilize them and find something of greater constancy and duration. A square form keeps space constant while differences in proportion and color change it. We are constantly stimulated by the unforeseeable flow of existence in everyday life and open up to innovation on the one hand while seeking to maintain the integrity of our established equilibriums on the other.
The waves of a sea
Between 1920 and 1942 the square form is a constant feature but in a state of continuous evolution. The square is always different in appearance but always the same, just as the waves of the sea are always new and different from one another but still made of the same water.
Nature and life still remain the primary source of inspiration for abstract art. The beauty of a flower is certainly a model to be examined and from which to learn. I am thinking of certain watercolors by Paul Klee, the enchanting fragrance of the natural colors, and the incredible wealth of forms that the world offers to our gaze. The ten thousand different lines that we see around us prove on closer examination to be a single interminable line, because in nature everything is different, manifold, infinite, and at the same time one.
The one and the multiple
In Lozenge with Yellow Lines Mondrian expresses in the simplest possible form the interconnection between one and multiple.
We see here a square defined by four sides but extending partially beyond the edges of the painting. The square in this lozenge has the same proportions as the canvas:

Lozenge with Yellow Lines, 1933,
Oil on Canvas, cm. 80,2 x 79,9
The four lines show a progressive increase in thickness as we move clockwise from the vertical on the right. The more important innovation is obviously the fact that, for the first time, the lines are no longer black but yellow. On observing this square and contemplating the differing thickness of the lines, we are faced with a unity undergoing transformation from one side to the other; a synthesis that already appears comparatively manifold in itself.
Lozenge with Yellow Lines goes to the heart of the problem: to show multiplicity in unitary form; to open up unity, i.e., the postulate of consciousness, to the changing aspect of nature and existence in time but without losing sight of it. This is a fundamental issue. The one and the many appear as antithetical realities in the human dimension; in actual fact, they are the same thing.
Everything is one just as every individual thing is a complex set of parts. the apparent simplicity of a leaf is a small universe and that the immensity of earthly nature is a bluish-white spot in the infinite space of the macrocosm. The immensity of earthly nature is as simple as a leaf, which is as complex as the entire planet. Multiplicity becomes unity and unity reveals multiplicity.
How to express this conception of reality except in abstract form?
The interplay between multiplicity and unity lies at the heart of Neoplastic new vision of reality and will find magnificent representation in the last work completed by Mondrian Broadway Boogie Woogie:

The above is not just an exercise of cold geometry for its own sake. “Art conveys the deepest thought by means of the simplest form.” (Albert Einstein)
For a detailed examination of the evolution process and for an explanation of its existential meanings visit: An Overview