San Marco Altarpiece main subject Madonna and Child
The San Marco Altarpiece (also known as Madonna and Saints) is a painting   by the Italian early Renaissance painter   Fra Angelico, housed in the San Marco Museum of Florence, Italy. It was commissioned by  Cosimo de' Medici the Elder, and was completed sometime between 1438 and 1443. 
In addition to the main panel depicting the enthroned Virgin and Child surrounded by Angels and Saints, there were nine predella panels accompanying it, narrating the legend of the patron saints, Saints Cosmas and Damian.  Only the main panel actually remains  to be seen in the Convent of San Marco, Florence, Italy, today, along with two side panels  depicting saints which were purchased back for the museum as recently as 2007. 
The San Marco Altarpiece is known as one of the best early Renaissance paintings for its employment of metaphor and perspective, trompe-l'œil, and the intertwining of Dominican religious themes and symbols with contemporary, political messages.Description
The San Marco Altarpiece depicts a portrait of the Virgin and Child seated on a throne surrounded by saints and angels. The formal elements are innovative for a contemporary Virgin and Child altarpiece as the positioning of the characters creates a deeply receding and logical space in front of the landscape background. The pomegranate embroidered curtain behind the Virgin and Child establishes a distinct horizontal line separating the events depicted in the painting from the landscape behind it. The altarpiece is situated on the then newly invented single rectangular panel, which helps turn a typical easel painting into the principal image of the altarpiece. Representing the figures set within a coherent pictorial space was also a new technique Angelico employed. While partially covered by the saints and angels, there is a definite line created by the carpet's receding squares in the foreground adding depth to the painting. Angelico's use of space is exceptional as he creates a sense of balance on both sides of the Virgin and Child, but also leaves available space on the carpet approaching the Virgin and Child so the viewer does not feel blocked or overwhelmed. This symmetry and order would allow worshippers to clearly view the painting from afar. 
He also employs naturalistic effects of light and color combined with a variety of colors and patterns. The natural colors contribute to the slightly darker complexion of the painting, which may accentuate the sacred holiness of the moment. His usage of the red and blue traditional colors of the Virgin and Saints Cosmas and Damian is noteworthy. By dressing both kneeling saints as well as several of the angels in red, Angelico creates a vertical link and further geometric stability. The symmetry resulting from the figures and colors allows the viewer to zoom-in and creates a smooth, continuous movement from figure to figure, eventually arriving at the Virgin and Child in the center.  On the right, Saint Damian kneels on an inward angle towards the center praising the Virgin and Child, which draws the viewer's eyes towards the painting's vanishing point at the Virgin's chin. The Virgin and Child are featured precisely at the vertical and horizontal axes' intersecting points and are placed above Angelico's trompe-l'œil depiction of the crucifixion.
One may criticize Angelico for his imperfect use of scale. Though they are sitting on a pedestal, the Virgin and Child do not seem much larger than the rest of the characters, showing a lack of a scale setting the main subjects apart from other mortals. If anything, the Virgin and Child should be smaller due to their increased distance from the viewer. While he does create a sense of depth, the distance to approach the throne is minimal, which some historians perceive as a lack of awe for the holy figures. The altarpiece is thus seen as a radical departure from the vulnerable models known in Dominican art. One should also note that the San Marco Altarpiece is one of the earliest examples of  sacra conversazione (sacred conversation), a type of image showing the Virgin and Child amongst saints in a unified space and single pictorial field, rather than setting them completely apart.Saints
The saints play an integral role in the structure and program of the altarpiece. The two patron physician saints, Saints Cosmas and Damian, are the most commented on subjects of the painting as the intercessors between the Virgin and Child. The saints are kneeling most immediately in the foreground, making them larger than the remaining figures and signifying their importance. Saint Cosmas is seen as the primary interlocutor figure as he mirror's the viewer's glance, looking directly at the viewer. This establishes a sense of accessibility to the painting on the viewer's part. Saint Cosmas's representation pays homage to Cosimo de' Medici since it has been identified as a portrait of Cosimo himself. On the right, Saint Damian takes the expected kneeling pose of a Dominican worshipper showing reverent devotion. His positioning, as written above, helps contribute to the viewer's visual path towards the center of the painting. If one looks carefully, Saints Cosmas and Damian form two bases of a triangle whose apex is the Virgin and Child. On the far left, Saint Lawrence, representing Cosimo's brother Lawrence, too glances out towards the viewer as an invitation into the holy space of the Virgin and Child. He also represents Cosmas' deceased brother, Lorenzo. Next to Saint Lawrence is Saint John the Evangelist standing for both Cosimo's father, Giovanni di Bicci de' Medici and Cosimo's own son, Giovanni de' Medici. Saint Mark, the dedicatee of the church, is seen next to Saint John the Evangelist holding an open codex above Saint Cosmas's head, which is further discussed below (Symbols). The right side of the painting features Saint Dominic, closest to the Virgin, and Saint Peter the Martyr, closest to the viewer,  as the 2nd Dominican saint depicted. The central saint on the right side is Saint Francis who stands for Cosimo's elder son Piero and most likely Lorenzo's son Pier Francesco.