Recently the Teaching & Technical Committee (TTC) was asked to clarify whether T-shirts, singlets or other similar garments, could be worn underneath
gi tops. For normal training in the dojo, the tradition is that men don’t wear anything under the gi top and that women do wear a T-shirt.
In that situation, a person should advise the new instructor before the class, that they are aware of the traditional dress code, and then give their reason for not being able to follow it. For both women and men, the T-shirt should ideally be white and plain, and not stand out underneath their gi.
6th Dan Shidoin