Do you have a device with a 3. 5mm plug that won't fit into your other device with a 2. 5mm jack such as a pair of standard headphones and a new smartphone? If so you need this adapter from Monoprice! This adapter has a mono 3. 5 mm female TS jack and a mono 2. 5 mm male TS plug. Note that this is a mono adapter. If you need this for a stereo audio application such as listening to music on your smartphone with your standard headphones/earbuds you should get the stereo version PID 7124 instead.