Its not so much that brass tacks are 'low' as that they are 'deep.' There are several theories on the etymology of the phrase. The one I adhere to comes from the uphostery trade. The webbing and other 'structurall' parts of the upholstery are fastened to the wooden frame of a chair with brass tacks. They are covered up with the padding, cushioning, and actual uphostery. To get down to brass tacks means to remove all the padding and 'fluff' ( hype and PR) and get down to the underlying basics. There is also a theory that sellers of dry goods (cloth) had brass tacks along the edge of the counter at one yard and other measurements, getting down to brass tacks meant taking actuall measurements against them instead of the ol' nose to finger one yard measurement. Orrr... In the shoe making trade brass tacks were used to fasten soles to shoes, getting down to B T, was filling the soles to show that the shoes were properly constructed, orrr when the sole wore down the tacks protuded into the wearers soles. Either way you were down to the bottom of things. The dumbest one I've seen refers to a supposed 'Brass Tax,' either in the US or England. I can't find out anything about such a tax or what it would have to do with the meaning. So how low the tacks are depends on if you are sitting in a Danish Modern or Early American chair.