The kirpan is a curved sword or dagger worn on a strap. Its length is not fixed by the Rehat Maryada, which says only that it should be worn.
It is the article that has produced almost every legal conflict involving Sikhs in western countries: schools, courts, aircraft, parliaments. Canada's Supreme Court held in 2006 that a Sikh student could wear one to school. Several jurisdictions have specific exemptions in their weapons legislation, and several do not.
The Indian Constitution, in the explanation to Article 25, expressly provides that wearing and carrying kirpans is included in the profession of the Sikh religion.
Doctrinally it is not a weapon of aggression. The traditional formulation is that it is drawn only in defence of the defenceless, which is the ethical position set out in the Zafarnama.
The five kakars together are wearable and the point of that is that a Sikh cannot pass. Whatever else the tradition did, it made concealment impossible.
It is worn on a gatra over the shoulder and the rehat requires it at all times. Its length is not fixed and practice runs from a few inches to a full sword.
It has been litigated repeatedly, in Indian courts, in Canadian schools, in British workplaces and in airports everywhere. The Indian constitution names it explicitly.
The argument the tradition makes is that the weapon is carried by somebody bound to use it only in defence and only for others. The obligation and the object are meant to be the same thing.