The flower spike is of numerous pale lilac or pink florets, and marked with purplish lines or spots., although other forms can be found from white to reddish-purple. The narrow leaves are usually spotted or blotched with brownish markings. It grows up to 70 cm tall, with flowers up to 2 cm wide.
It flowers June to July. Found in open woods, meadows, fens and marshes, on roadside verges and railway banks. Locally abundant and widespread throughout Britain, except in Devon and Cornwall, and the north-east of Scotland.