One may be perplexed about the voltage across an inductor, since sometimes it is -LdI/dt, but more commonly it is LdI/dt. We explain why this is so, and point out that in fact, each of them is fine to be used if under its own sign convention (listed in table 2 in the last section of this paper), and the latter, mostly used in circuit equations, is for the purpose of intentionally grouping L, C and R as passive components (consuming electric potential energy), enabling circuit analyses more conveniently and results more understandable. As well, we re-examine a closely related subject matter: the determination of the polarities in an a.c. circuit.