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, even pure white. Each flower appears to have 4 lobes, two upper (which are generally paler) 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.