Underdrawing is the drawing

Drawing

Drawing is a form of visual art that makes use of any number of drawing instruments to mark a two-dimensional medium. Common instruments include graphite pencils, pen and ink, inked brushes, wax color pencils, crayons, charcoal, chalk, pastels, markers, styluses, and various metals .An artist who...

 done on a painting ground

Ground

Ground may refer to:* Earth's surface* Soil, a mixture of clay, sand and organic matter present on the surface of the Earth and serving as substrate for plant growth and micro-organisms development...

 before paint is applied, for example, an imprimatura

Imprimatura

Imprimatura is a term used in painting, meaning an initial stain of color painted on a ground. It provides a painter with a transparent, toned ground, which will allow light falling onto the painting to reflect through the paint layers. The term itself stems from the Italian and literally means...

 or an underpainting

Underpainting

In art, an underpainting is an initial layer of paint applied to a ground, which serves as a base for subsequent layers of paint. Underpaintings are often monochromatic and help to define colour values for later painting...

. Underdrawing was used extensively by 15th century painters like Jan van Eyck

Jan van Eyck

Jan van Eyck was a Flemish painter active in Bruges and considered one of the best Northern European painters of the 15th century....

 and Rogier van der Weyden. These artists "underdrew" with a brush, using hatching strokes for shading, using water-based  black paint, before underpainting

Underpainting

In art, an underpainting is an initial layer of paint applied to a ground, which serves as a base for subsequent layers of paint. Underpaintings are often monochromatic and help to define colour values for later painting...

 and overpainting

Overpainting

Overpainting can mean the final layers of paint, over some type of underpainting, in a system of working in layers. It can also mean later paint added by restorers, or an artist or dealer wishing to "improve" or update an old image—a very common practice in the past. The underpainting gives a...

 with oils. Cennino D'Andrea Cennini

Cennino D'Andrea Cennini

Cennino d'Andrea Cennini was an Italian painter influenced by Giotto. He was a student of Agnolo Gaddi. Gaddi trained under his father, called Taddeo Gaddi, who trained with Giotto. Cennini was born in Colle Val d'Elsa, Tuscany....

 (14th century most likely) describes a different type of underdrawing, made with graded tones rather than hatching, for egg tempera.

In some cases, underdrawing can be clearly visualized using infrared

Infrared

Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 reflectography because carbon black

Carbon black

Carbon black is a material produced by the incomplete combustion of heavy petroleum products such as FCC tar, coal tar, ethylene cracking tar, and a small amount from vegetable oil. Carbon black is a form of amorphous carbon that has a high surface-area-to-volume ratio, although its...

 pigments absorb infrared light, whereas opaque pigments such as lead white are transparent with infrared light.

What is remarkable about some historical examples of underdrawing, in, for example, the Annunciation (van Eyck, Washington)

Annunciation (van Eyck, Washington)

The Annunciation is an oil painting by the Early Netherlandish master Jan van Eyck, from around 1434-1436. It is in the National Gallery of Art, in Washington D.C. It was originally on panel but has been transferred to canvas. It is thought that it was the left wing of a triptych; there has...

 or the Arnolfini Portrait

Arnolfini portrait

The Arnolfini Portrait is an oil painting on oak panel dated 1434 by the Early Netherlandish painter Jan van Eyck. It is also known as The Arnolfini Wedding, The Arnolfini Marriage, The Arnolfini Double Portrait or the Portrait of Giovanni Arnolfini and his Wife, among other titles...

, is the extent to which artists made radical alterations to their carefully laid designs.