Picture a very long table with only five or six chairs at it, and a large crowd standing outside the door. Everything that matters is decided at that table, and the door does not open. A useful way to hold this word is the gap between how few chairs there are and how many people have to live with what is decided on them.
The packet gives the word as Greek oligos, few, plus arkhe, rule, reaching English through Late Latin and French, and notes the surface analysis oligo- plus -archy. The few and the ruling are both written into the word.
證據裡寫得很清楚：來自希臘文 olígos（少數）加 arkhḗ（統治），經 Late Latin 和法文進入英文；表面上也可以拆成 oligo- 加 -archy。「少數」和「統治」兩塊都直接寫在字裡面。學會這兩塊，意思幾乎不用背。
Wiktionary (kaikki extract) via evidence packet: oligarchy, etymology_text (Greek oligos + arkhe; surface analysis oligo- + -archy)
Two branches
那一小群人，和他們統治的地方
英文用同一個字指桌邊那幾個人，也指被他們管的那個國家或團體，因為在說話的人眼裡，兩者已經分不開了。
noun · C1
A small group of people who hold the power in a country or an organization, or a country that is run in this way. The group is usually rich or from a few powerful families, and ordinary people have little say.
寡頭集團；寡頭國家
那一套制度本身
把注意力從桌邊的人移開，只看「只擺幾張椅子」這個安排本身，就是制度的意思。
noun · C1
The system in which a country or an organization is run by a small group of people, rather than by everyone together or by a single ruler.
寡頭政治（制度）