Common logarithm
Before the early 1970s, hand-held electronic calculators were not yet in widespread use. Because of their utility in saving work in laborious calculations by hand on paper, tables of base-10 logarithms were found in appendices of many books. Base-10 logarithms were called common logarithms. Such a table of "common logarithms" gave the logarithm of each number in the left-hand column, which ran from 1 to 10 by small increments, perhaps 0.01 or 0.001. There was no need to include numbers not between 1 and 10, since if one wanted the logarithm of, for example, 120, one would know that- <math>\log_{10}120=\log_{10}(10^2\times 1.2)=2+\log_{10}1.2\cong2+0.079181.</math>
Common logarithms are sometimes also called Briggsian logarithms after William Briggs[?], a 17th-century British mathematician.
Because base-10 logarithms were called "common", and engineers often had occasion to use them, engineers often wrote "<math>\log(x)</math>" when they meant <math>\log_{10}(x)</math>. Mathematicians, on the other hand, wrote "<math>\log(x)</math>" when they mean "<math>\log_e(x)</math>" (see natural logarithm). Today, both notations are found among mathematicians. Since hand-held electronic calculators are designed by engineers rather than mathematicians, it became customary that they follow engineers' notation. So ironically, that notation, according to which one writes "<math>\ln(x)</math>" when the natural logarithm is intended, has been further popularized by the very invention that made the use of "common logarithms" obsolete: electronic calculators.
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