*sed-

The core idea of this root was

sitting

Today, the majority of members of the family are about a somewhat broader sense of “putting down”.
Here’s a little (incomplete) overview:

• to sit, seat (“sitting”)
• saddle (“seat”)
• to set (“putting down”)
• settle (“sitting down for camping”)
• soil (“where your settlement is”)
• session (“the sitting”)
• seance (“the sitting”)
• siege (“sitting in front”)
• sediment (“dust settling”)
• possess (“sit on”)
• obsess (“sit on, occupy”)
• sedate, sedative (“settling”)
• assist, assistent (Originally, a person sitting next to the judge)
• assess (see “assist”, originally an assistant responsible for determining fines)
• size (see “assess”)
• cathedral (Originally, it was a scholar’s chair, the root is the “thed”- part)
• chair (comes from cathedral)