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Is there any glyph difference between 'Em dash' (— U+2014) and the 'Horizontal bar' (― U+2015)?
There are a number of dashes encoded on Unicode, but this is one of the few that is not very clear its usage. Unicode calls the horizontal bar 'quotation dash', yet is not clear why it was encoded differently, as all bibliography I've read makes no distinction between the dash used for parenthetical text and quotations. Fonts I've seen makes horizontal bar longer, shorter or both characters the same. Recurrently the em dash connect to the sides of the character width, but the horizontal bar not. A difference in the characters properties is that em dash makes line breaks while horizontal bar don't—which may make sense as distinguishing them semantically, but don't give much hints for the glyph side.
I probably won't be very interested by this, but the horizontal bar is included in an old Windows character set for Greek, along Em dash. I'm not sure if is used down there or not.