A common plant, with distinctive interrupted whorls of 3–5 flowers giving the plant a somewhat pagoda-like appearance. The flowers are typically pink-purple but are quite variable, sometimes even pure white. Each flower appears to have 4 lobes, two upper (which are generally paler than the lower) and two lower. In fact there is a third lower lobe, hidden between the other two, keeled and enclosing the stamens and style. The leaves are lance-deltate, opposite and sessile on the straight stem.