Articles on ideas in the history of mathematics notation?

One particularly intriguing piece of history of the sign for equality is Leibniz's use of the sign "$_{\ulcorner\!\urcorner}$" for equality, rather than the "=" we are familiar with. On the other hand, Leibniz emphasized repeatedly that his was a generalized relation of equality "up to" an infinitesimal, so that one could have $a +dx \;{}_{\ulcorner\!\urcorner} \;a$ for nonzero real $a$. This piece of notation is mentioned in an article by McClenon, R. B.: A Contribution of Leibniz to the History of Complex Numbers. American Mathematical Monthly 30 (1923), no. 7, 369-374 online here. For a related discussion of Leibniz, see the recent article here.

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