In late 1958, Julio Le Parc arrived in Paris courtesy of a grant from the French government, at a time when kinetic art was all the rage and a considerable number of Latin American artists had flocked to the city. The following year he began to work regularly with a creative system based on sequences and progressions in the context of the Groupe de Recherche d'Art Visuel (GRAV), which he launched with Francisco Sobrino, the Frenchman François Morellet and other artists. He juggled his activity within the group, which endured until 1968, with his own personal work, unveiled at his first exhibition in New York in 1966 and recognised that same year with the Grand Prize for Painting at the Venice Biennale.

Le Parc's most important contribution to art in the 1960s and 70s was his use of a unitary system to tackle form on the pictorial surface, moving past both Abstract Expressionism (called Tachisme in France) and geometric abstraction. The basis of his approach was a method of progression that allowed him to uniformly cover the entire pictorial surface, yielding two important series of surface works which he called Surface-Colour and Modulations. The former used a limited range of fourteen colours, from lemon yellow to orangey-yellow, with greens, blues, maroons and oranges in between, to produce flat, uniform paintings based on combinations of simple geometric shapes (bands, squares or circles) according to predetermined colour progressions.

The Modulations series exhibited in 1976 at two galleries, Denise René in Paris and Rayuela in Madrid, was a consequence of the Surface-Colour series. In these works Le Parc brought volume and undulating movement to the picture plane, maintaining the same palette of fourteen colours and applying them with an airbrush to achieve a gradient effect, an effect heightened by the waving motion of the colour planes. The Modulations complemented his other investigations in the 1960s, such as his research into contorted and luminous forms. They also coherently paved the way for the rest of his three-dimensional work—his reliefs, mobiles, works with pulsating light, arcades, etc.—in which he maintained the contorted forms and incorporated real light and spatial experimentation.

Modulación n.º 66 [Modulation no. 66] is a vertically oriented work that deploys the middle range of the fourteen-colour spectrum, where the cooler tones are situated. As the artist himself noted, these "modulations are still founded on simple systems of organisation, and the correlation of forms relies on the same principle in every case", but here he has abandoned the recourse to peripheral vision seen in his early works. His Modulations, which disconcerted critics because of their seeming reliance on trompe l'oeil, are in fact based on an interest in experimenting with simple organisational systems and on the correlation of forms. These airbrushed pieces are connected, according to the artist, to his earliest works and would dominate his production until the mid-1980s. [Carmen Fernández Aparicio]