By inquiring about clavus on the Net, now I can confirm that the horn plug under the skin of the sole of my left foot belongs to one of its symptoms. Prolonged standing and walking can make people take the disease, which is the result of friction and pressure. Wearers of tight and narrow shoes or people with deformed polarthums are likely to suffer friction and pressure on the feet, which can thicken the cuticula in the spot concerned, growing inward like a circular cone. The dimension of the diseased part is just like a pinpoint or a broad bean. It pains the patient when he walks about.