【Harmony】 ,  【consonance】 ,  【accord】 ,  【concord】  all designate the result attained or the effect produced when different things come together without clashing or disagreement.

【Harmony】 basically denotes the unity, order, and absence of friction produced by the perfect articulation and interrelation of distinct parts in a complex whole.

Largely because of its associations with music the term often suggests beauty of effect, whether achieved by order in a whole or by agreeable blending or arrangement (as of tones, colors, and features).

【Consonance】  may denote the blending of two or more simultaneous sounds so as to produce an agreeable effect and is then opposed to dissonance. More often it names the fact or the means whereas  【harmony】 names the result.

In extended application  【consonance】 commonly retains these implications of coincidence and concurrence. Sometimes the pleasurable effect is still suggested.

Very often there is no hint of a pleasurable effect but a strong implication of consistency or congruity.

【Accord】  is often interchangeable with  【consonance】 without loss.

However, it can imply, as  【consonance】 cannot, personal agreement or goodwill or, often, absence of ill will or friction. Hence the phrase "with one  【accord】 " suggests unanimity.

【Concord】 , like  【accord】 , often stresses agreement between persons but is more positive in its implications, for it suggests peace with amity rather than absence or suppression of ill will.  【Concord】  also, when applied to sounds, comes close to  【harmony】 in its implications. However it seldom except in a technical sense in music connotes 【consonance】 of tones, but usually the pleasant succession of tones that is the quality of melody.