Because of the softness of pure (24k) gold, it is usually alloyed with base metals for use in jewelry, altering its hardness and ductility, melting point, color and other properties. Alloys with lower caratage, typically 22k, 18k, 14k or 10k, contain higher percentages of copper, or other base metals or silver or palladium in the alloy. Copper is the most commonly used base metal, yielding a redder color.
Basically using 24 as the pure base, each lesser 'k' (22, 18, 14 and 10) would be subtracted from the base (24) although it may be described as percentages such as 25% alloy 75% gold, etc. But for simplicity you'd compare the 22, 18, 14 and 10 from 24 (pure).
The main difference is the Gold content (14 kt contains more gold than 10 kt and thus is more expensive). There is a slight difference in the mechanicqal properties too.
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