The common logarithm that is a logarithm with base 10 i.e. log_10 2, log_10 3. It is so popular that leave 10 for shorten notation i.e. log_10 2 = log 2, log_10 3 = log 3. Based on definition of logarithm: log 1 = 0, log 1/10 = -1 & log 10 = 1, log 1/100 = -2, log 100 = 2, log 1/1000 = -3, log 1000 = 3, log 1/10000 = -4, log 10000 = 4. Using commons logarithms and formula log_a b = log_c b / log_c a can calculate logarithm with any base i.e. log_3 7 = log7 / log3 ≈ 0.85 / 0.48 ≈ 1.77. This way is useful if a calculator has only key with a common logarithm.