Bachelor's degree
Until the 19th century, a bachelor's degree represented the first degree in a particular faculty, with Arts representing undergraduate study, thus the Bachelor of Civil Law (BCL) at Oxford and the Bachelor of Laws (LLB) at Cambridge, for example, were postgraduate degrees. Vestiges of this system still remain in the ancient universities, with Oxford and Cambridge awarding BAs for undergraduate degrees in both arts and sciences (although both award undergraduate BTh degrees through associated theological colleges, and Oxford awards BFA degrees in addition to the BA) and defining other bachelor's degrees (e.g. BPhil, BCL) as postgraduate awards equivalent to master's degrees, although many postgraduate bachelor's degrees have now been replaced by equivalent master's degrees (e.g. LLM for the LLB at Cambridge and MSc for the BSc at Oxford). The same historical usage of indicating an undergraduate degree by it being in the faculty of arts rather than being a bachelor's degree gives rise to the Oxbridge MA and the Scottish MA).
British degree abbreviations
Conversely, some bachelor's degrees in the "higher faculties" at the older universities in the UK (e.g. those other than arts at Oxford and Cambridge) are postgraduate qualifications (e.g. the BCL and BMus at Oxford). Many have been changed to the corresponding master's degree (e.g. BSc is now MSc at Oxford), but only within the last generation. The BD (Bachelor of Divinity) remains a higher degree at some universities (e.g. Oxford, Cambridge, St Andrews and, until recently, Durham) but is an undergraduate degree at most (e.g. London, Edinburgh, Aberdeen and Glasgow).