selo

The s / ŝ / ĉ chain, and it is worth naming. Four Esperanto words sit within one consonant of each other and all four are common enough to meet:

• selo - a saddle, this page, rank 2,300
• ŝelo - a peel, rind, shell or bark, rank 2,071
• ĉelo - a cell, of a body, a prison or a honeycomb, rank 1,911
• celo - a goal, aim or target, rank 287

Four different roots, four different origins, one vowel and one ending in common. There is no trick and no shared etymology to hang them on; the only defence is to hear the initial consonant properly and to write the hat when it is there. This is the strongest practical argument on the site for learning the three letters as three sounds rather than as decorated versions of s and c: s as in sit, ŝ as in shut, ĉ as in church. Say ŝelo and ĉelo aloud until they stop sounding like variants of each other, because in running text the reader has only that one letter to go on.

A related trap for anyone typing in the x-system: sxelo and selo differ by two characters that sit next to nothing memorable, and a dropped x turns bark into a saddle silently. The orthography and punctuation article covers the h-system and x-system and when each is appropriate.

Using it. Seli is to saddle a horse, senseligi to unsaddle, selisto a saddler. Esperanto uses the same word for a bicycle saddle, as English does, and en la selo covers both the literal seat and the figurative sense of being in charge. The -ejo you might expect for a saddler's shop is selejo, though selisto covers the trade well enough that you will rarely need it.