-ec-

-ec- lifts a quality out of the thing that has it and names it. Bela gives beleco, libera gives libereco, bona gives boneco, juna gives juneco, amiko gives amikeco. The result is a noun in -o, and eco stands alone meaning a property or characteristic.

The contrast to keep straight is with -aĵ-, which names the concrete thing instead: beleco is beauty, belaĵo is a beautiful object you could point at. Confuse the two and you compliment the vase when you meant to praise the idea.

One practical note. Plenty of adjectives already have a plain noun sitting beside them, and the plain noun usually wins. Varmo is heat and varmeco is warmth-as-a-property, which is rarely what you want; reach for -ec- when no simpler noun exists.