Up until 1508, Secular Franciscans wore full habits. Then this changed to a large scapular held together by a piece of rope around the waist. Over time, the scapular became smaller until the rope was no longer needed to hold it in place and was was replaced by a string, and both were then worn underneath clothing.
Humility, Holiness and Charity.
With the revision of our Rule in 1978, our habit changed from the small scapular and string to — in the U.S., at least — the Tau cross. It is most often found wooden, with a knotted cord. The three knots symbolize the religious solemn vows of Poverty, Chastity and Obedience. Seculars don’t make vows, but make a lifetime profession to live Gospel to Life and Life to Gospel practicing the secular version of Poverty Chastity and Obedience which are Humility, Holiness and Charity.