A bertha is a collar made of lace or another thin fabric. It is generally flat and round, covering the low neckline of a dress, and accentuating a woman's shoulders. It was particularly popular in the nineteenth century, but bertha-like garments were also worn in the seventeenth century and are still being worn, as for instance with bridal dresses.
The word was first written down in English in 1856. According to the Shorter Oxford Dictionary, the word is an anglicised version of the French berthe, which is a deep falling collar attached to the top of a low-necked dress.
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Digital source of illustration (retrieved 8 July 2016).