Logarithmic scales are either defined for *ratios* of the underlying quantity, or one has to agree to measure the quantity in fixed units. Deviating from these units means that the logarithmic measure will change by an *additive* constant. The base of the logarithm also has to be specified.

- Richter Magnitude Scale for strength of earthquakes
- bel and neper for acoustic power (loudness) and electric power
- cent for the relative pitch of notes in music
- logit for odds in statistics
- Palermo Technical Impact Hazard Scale
- counting Nines for probability
- counting f-stops for ratios of photographic exposure
- pH for acidity
- Stellar magnitude scale for brightness of stars

In the last two examples large values (or ratios) of the underlying quantity will correspond to negative values of the logarithmic measure, because of reversal of the scale by a minus sign in the definition.