2.5mm TRS
Direct Answer
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.
Source: EIA RS-453 / IEC 60603-11
Specs
| Shaft Diameter | 2.5 mm |
| Conductors | 3 (Tip, Ring, Sleeve) |
| Typical Wiring (Unbalanced Stereo) | Tip: Left | Ring: Right | Sleeve: Ground |
A Smaller Size for Compact Devices
2.5mm TRS follows the exact same tip-ring-sleeve wiring logic as its larger 3.5mm and 6.35mm counterparts -- it's simply a smaller shaft diameter, used where device size mattered more than compatibility with standard headphone plugs. Documented uses include headset jacks on cordless phones and two-way radios, and remote on-off switch jacks on early portable tape recorders (sitting alongside a separate 3.5mm microphone jack on the same device), especially in the years before 3.5mm became the near-universal mobile headset size.