2.5mm TRS vs. 3.5 mm TRS

2.5mm TRS is the smallest common member of the phone-connector family -- the same tip-ring-sleeve wiring as 3.5mm TRS, scaled down to a 2.5mm shaft. It was used for headset jacks on cordless phones, two-way radios, and early portable tape recorder remotes before 3.5mm became the standard smaller size.

3.5mm TRS is the 3-conductor version of the 3.5mm mini-jack. It's wired either as unbalanced stereo audio (tip: left, ring: right, sleeve: ground) in consumer headphones, or as balanced mono (tip: hot, ring: cold, sleeve: ground) in some professional audio gear -- the same three contacts, different signals depending on the device.