cad/cas/cid

fall

Spelled cad, cas, cid · cadere 'fall'; cas- appears in casus, and cad- becomes cid- in compounds

Latin cadere meant fall: a cascade carries falling water, cadence follows a voice's rise and fall, and decadent suggests decline.

Words built from it

cadence — the rhythmic flow or rise and fall of sound
cadere (fall) → Latin cadentia (a falling) → Italian cadenza → cadence
cascade — a waterfall or something flowing downward in stages
Latin cadere (fall) → Italian cascare (fall) → cascata (waterfall) → French cascade
decadent — marked by excessive luxury or declining standards
de- (down) + cadere (fall), through Medieval Latin decadentia (decline) and French décadent
incident — an event or occurrence, especially an unusual one
in- (upon) + cadere (fall) → incidere (fall upon, happen) → incident- (happening)
accident — an unintended event, often causing harm
ad- (upon) + cadere (fall) → accidere (happen) → accident- (happening)
coincide — happen at the same time or agree exactly
co- (together) + in- (upon) + cadere (fall) → coincidere (fall together)
occasion — a particular event, time, or opportunity
ob- (down, against) + cadere (fall) → occidere (fall down) → occasio (opportunity)

casket — Contains cas, but comes through French cassette, a small box, ultimately from Latin capsa (box).
acidic — Contains cid, but comes from Latin acidus (sour), related to acere (be sour), not cadere.