Efficiency and effectiveness. I've dealt with these words before (efficiency refers to process, how well a system functions in itself, whereas effectiveness refers to product, how much impact a system generates), but this time I want to focus on the word endings. Some words end only in -ency, others only in -ence, and some take both endings. The difference could be that -ence indicates the state ("stav") of something, like "dependence on drugs" (it's not relative - people either have it or they don't), whereas -ency indicates the quality or the measure ("miera") of something, like efficiency. There's also "a dependency", which means a self-governing island that belongs to the British crown (the Channel Islands and the Isle of Man), as opposed to "United Kingdom overseas territories" such as the Falkland Islands (so this could be an issue of quality, or the measure of their independence). Probably a better example is "urgency" - the relative measure of how urgently something needs to be done.

The second issue is that some words end only in -ness, others only in -ity, and some take both endings. Again, the difference seems to be between quality and state. Effectiveness refers to the quality of a system in terms of measuring the effect it produces. Effectivity means the state (condition) of a law which is in effect (it's not relative - the law is in force or it isn't). Another example is "infinity", the mathematical limit or constant expressed with the symbol , compared with "infiniteness", which expresses the quality of space or the cosmos, our impression or feeling about the universe. Of course this is not an absolute rule - consider intelligence and stupidity: are they "have it or not" states, or are they "more or less" measures?