is native to Europe and Asia but is now found in most parts of the world. Stems are tall, stout, ridged, square in cross-section and lightly covered by short hairs. Stems can be simple or branch a few times. Leaves are quite variable, generally becoming smaller, narrower and simpler further up the stem. The lowest are broadly lanceolate in shape, usually with five lobes, lined by a few teeth, while the uppermost are narrowly ovate and have just two small lobes, or teeth. Leaf surfaces are crossed by prominent, branched veins.
Flowers form in whorls just above the leaves, along the upper half of the stem. Flowers have a rigid green calyx, extending to five narrow, spine-tipped lobes, and a tubular, white or pale pink corolla, opening to two lips. The upper side of the corolla is covered by white hairs. The lower lip is marked by a few purple dots, and is divided into a larger central lobe and a pair of smaller side lobes.