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! Important: Although this adapter will physically connect a headset to a smartphone headset/microphone jack this will not allow use of the microphone. This adapter has TRS (Tip-Ring-Sleeve) connectors with three contact points while a headphone/mic device requires TRRS (Tip-Ring-Ring-Sleeve) connectors with four contact points