harmonise vs sing

harmonise and sing both are verbs.

Word	Noun	Adjective	Verb	Adverb
harmonise	No	No	Yes	No
sing	No	No	Yes	No

As verbs, sing is a hypernym of harmonise; that is, sing is a word with a broader meaning than harmonise:

harmonise (verb)	sing (verb)
bring into consonance, harmony, or accord while making music or singing	make a whining, ringing, or whistling sound
bring into consonance or accord	divulge confidential information or secrets
bring (several things) into consonance or relate harmoniously	to make melodious sounds
sing or play in harmony	produce tones with the voice
write a harmony for	deliver by singing
go together