ĥoro

This is the word that keeps ĥ alive. Esperanto has spent a century quietly retreating from its rarest letter, and the escape route was always the same: swap ĥ for k, which is made in the same place in the mouth. Ĥemio became kemio, meĥaniko became mekaniko, arĥitekturo became arkitekturo, and the Akademio ratified the lot in 1974. It could not do it here. Ĥoro cannot become koro, because koro is a heart, and a language cannot have the church choir and the thing in your chest spelled identically. So the alternative had to be built somewhere else entirely: koruso, back to Latin chorus, with a whole extra syllable. The same block hit ĥolero, which had to go to kolerao because kolero is anger. The full story, the Akademio's list, and the rest of the surviving ĥ words are in the ĥ question.

Four words within two letters of each other, and three etymologies.Kordo opened this and it can be closed here. Koro is a heart, from Latin cor. Akordo is agreement, and the musical chord as a metaphor on top of it, built on that same Latin heart: ad cor, to the heart. Kordo is a string, from Greek khorde, a gut, which is what the first strings were made of. And ĥoro is the choir, from Greek khoros, a dancing-place and then the people dancing in it, related to none of the other three. Two of the four are relatives; the other two are strangers who happen to look alike.

Do not substitute plain h. Ĥoro and horo are different words, and horo is an hour, which you will say fifty times as often. The sound you want is the ch of Scottish loch. In use: a ĥorestro conducts, a ĥorano sings in it, ĥorala is the adjective for chorale music, and the whole ensemble is what turns up beside the orkestro at a koncerto.