Obi is theoretically not necessary for a coat which opens in front, and it is enough to provide a cord on each side to tie so as to hold the clothes secure and prevent them from opening, like in the case of gown.
前開きの上着に対して帯は原理的には必須ではなく、ガウンなどのように、脇の部分に結ぶための紐を備えることで、開かないようにできる。