under-bæc
This might be just a supplemental entry adding to an entry in the Main Volume.
Related §§ in Wright's OE Grammar:
In the OE text, the length is:indicated by acute accentsindicated by macronsnot indicated.
under-bæc
. Ib. add: — Ðás seofon tunglan gáð ǽfre eástweard ongeán þá heofenan, ac seó heofon is strengra and ábrét hí ealle under-bæc westward mid hire ryne, Angl. vii. 14, 137. II. add: — Hit is áwriten ðæt him wǽre betere ðæt hí nó sóðfæstnesse weg ne ongeáten, ðonne hí underbæc gecerden siððan hí hine ongeáten, Past. 445, 33.
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A Combined List of Abbreviations.

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A Combined List of Abbreviations.