-estr-

-estr- gives the person in charge of the thing the root names. urbo gives urbestro, ŝipo gives ŝipestro, lernejo gives lernejestro, regno gives regnestro. The result is a noun in -o, it takes -in- for a woman, and estro stands alone as the ordinary word for boss or chief.

Keep it apart from -an- and -ist-. An -estro runs the group, an -ano merely belongs to it, and an -isto practises the trade, so a school has one lernejestro and many instruistoj. The root must be the thing led, never the activity: a head cook is a kuirejestro, the head of the kitchen, and it is fenestro that trips everyone, being one plain root meaning window.