1911 Encyclopedia Britannica
Or Back-Letter, in Scots law, a deed qualifying the terms of another deed, or declaratory of the purposes for which another deed has been granted. Thus an ex facie absolute disposition, qualified by a back-bond expressing the limited nature of the right actually held by the person to whom the disposition is made, would constitute what in England is termed a deed of trust.
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Chisholm, Hugh, General Editor. Entry for 'Back-Bond'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/b/back-bond.html. 1910.