Capable of being harmonised or arranged in harmony; able to be brought into agreement or made to work together.
From harmonise (British spelling of harmonize: harmonium + -ise suffix) + -able (adjective suffix meaning capable of). The British -ise suffix reflects the word's development in British English.
In music, not every melody is 'harmonisable'—some note combinations are so weird or unusual that adding harmonies makes them worse, which is exactly why experimental composers deliberately choose non-harmonisable melodies!
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