Intro to Logarithms Page 3 of 3
Common Logarithms  Base 10A logarithm can have any positive number as the base. One of the most common bases for a logarithm is base 10. When a logarithm has a 10 as the base, it's called a common logarithm. If a logarithm is written without a base  without a little subscript number written  you can assume that the base is 10.
Most calculators have a button for a common logarithm. Look for the "log" button on your calculator. If you type in log(100), it should say the answer is 2 because 10 to the second power equals 100. 

Natural Logarithms  Base e
Another number that's commonly used as the base of a logarithm is the number e. e is an irrational number that comes up so often that it gets it's own symbol (similar to how the number 3.14159. . is called pi). The constant e is named after the mathematician Leonhard Euler. It is approximately equal to 2.718. When a logarithm has a base of e, it's called a natural logarithm. This is abbreviated as ln (that's a lowercase L. l for log, n for natural).
Most calculators also have a button to calculate a natural logarithm. Look for the "ln" button on your calculator. Remember, it's a lowercase L for log and the letter n to indicate a natural log.
Practice
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Interested in learning more about logarithms? Check out the lessons on solving equations with logarithms, logarithm rules, or the change of base formula.