Dear Sir or Madam, Your message has been received by Sony Customer Service, Japan. Thank you for your E-mail message. Firstly, we sincerely appreciate your patronage of Sony products for years. In reply to your inquiry, our comment is as follows, We avoid the repetition of the same model number of similar line of other companies and have decided the model number of the product. When the headphones were released at first, the model number had the acronym "DR-" (it means "[b]Dynamic Receiver[/b]"). And WALKMAN came out, the model number of headphones were changed to "MDR-" (we guess M stands for "[b]Micro[/b]" or "[b]Music[/b]"), and it came to be able to carry to the outside. Regrettably we could not find the concrete reason. Please understand. We hope you would kindly understand the above. Also, your continual attention to Sony products would be highly appreciated. Sincerely yours, Sony Customer Service(Japan) Inc., SCSJ