My background:

I just bought a new MacBook Pro with Thunderbolt 3 ports built in, and I have an old 12inch MacBook with a USB-C port. I was trying to use target disk mode with migration assistant to transfer data from my 12inch Macbook to my new MacBook Pro. I searched online and saw somewhere people say that Thunderbolt 3 cable can be used to do the job, moreover, there is information stating a Thunderbolt 3 can be used as USB-C cable as well, so I ordered Belkin Thunderbolt 3 cable on Amazon.(https://www.amazon.com/Belkin-USB-IF-Certified-Thunderbolt-Compatible/dp/B0725DW6D3/ref=sr_1_4?ie=UTF8&qid=1511766135&sr=8-4&keywords=belkin%2Bthunderbolt%2B3&th=1).

I received the cable today. However, by directly connecting my new MacBook Pro and MacBook 12inch, the new MacBook Pro couldn’t recognize that 12inch MacBook. I did some more research later, and on this page(https://support.apple.com/en-us/HT201462 scroll down to the bottom), it seems that a USB-C 3.1 cable is the only way that does the work.

I am quite confused since couldn't a thunderbolt3 cable be used as a USB-C 3.1 cable? Or, Apple just made it that way that the USB-C port on 12inch MacBook cannot recognize a Thunderbolt 3 cable? After this experience, I really want to understand the differences between USB-C and Thunderbolt3. By watching a lot of youtube videos explaining those differences, it brought to me even more questions, which are what I am asking here...

My questions:

So almost all the videos explaining Thunderbolt 3 and USB-C include USB A and USB B as well, and there is 1.0, 2.0, 3.0...version numbers too. Based on what I saw and my understanding, seems like the A, B, C types and those version numbers evolve separately.

  1. Does USB type letter A, B and C refer to shape only? Because all type A(no matter it is 1.0, 2.0 or 3.0) are the usual shapes of USB port we see and type B are those unusual shapes of ports we see, and type C is the one we see with PixelBook and other latest devices.

  2. Does version numbers refer to speed only?

  3. What's the relationship between USBC and Thunderbolt 3? Does Thunderbolt 3 fall into the category of USBC or, Thunderbolt 3 just "borrows" a USBC shape and in the meanwhile, it is a superset of USB 3.1 as well?

  4. Somebody says USB-C is USB 3.1. Is it correct? Since if a letter only refers to shape and number refers to speed, then there shouldn't be an equivalent.

Please advise... Thanks guys!

  • You ask multiple question in one post. That is going to make answering harder. You might want to split it into multiple posts. – Hennes Nov 27 '17 at 8:57
  • Re point 1: Yes. Shapes. USB A is the original plug for in the computer (master), and USB B is the large square plug for in the target device. These are the connector/forms factors for plugs and are independant of USB 1 or USB2. USB-C on the other hand refers to a new small plug which can be inserted both ways. The plug is mostly used for USB 3 gen 2, but thunderbolt shares the same plug. – Hennes Nov 27 '17 at 8:58
  • @Dave thanks for your reply Dave! I did... a lot, and every post and YouTube video even makes me more confused. I went to Intel Thunderbolt official website trying to find the answer, but I am still confused about its relationship with USB-C. It doesn't explain that much. Also, I couldn't find an answer on google that's directly related my questions, like what on earth do those types and version numbers represent... That's why I'm asking here. – Hang Nov 27 '17 at 8:59
  • And confusingly enough. Most (all?) thunderbolt implementations can fall back to USB mode. Which is great if you need fewer cables. And which is confusing as heck for most people. Especially if you add in DP multiplexing. – Hennes Nov 27 '17 at 8:59
  • @Hennes Thanks Hennes! Originally I wanted to put them in multiple different posts but since they are closely related, I think it would be hard to explain the reason behind why I am asking those questions. Anyway, your answers cleared a lot for me! Just one question about your point: what exactly do you mean by "fall back to" in "thunderbolt implementations can fall back to USB mode"? Does it mean Thunderbolt is not USB but it just borrows some USB implementation? – Hang Nov 27 '17 at 9:19

As noted in the comments, the question covers a lot of territory. A good starting place to get your arms around the subject of USB is this excellent Wikipedia article, including the related subjects it links to. But here's the gist.

You're on the right track, but not quite there. The numbers do reflect speed differences, but those differences result from the version standards that the numbers represent.

USB has gone through a series of development and refinement steps. Each release is defined by a comprehensive standard that covers everything from connector and cable definitions to protocols, etc. The number, like 3.1, is the release number of the standard. All of the elements defined by that release provide the ability to deliver the design speed and features associated with that release. A specific speed target is usually the main goal of a release.

The letters describe the connectors, or in some cases, the connector body. The different connector types have different intended purposes. For example, type A is typically used at the host end and type B at the peripheral end.

The connectors aren't uniquely associated with USB version numbers, but successive releases of the USB standards have introduced new connectors, and the connectors are fully compliant with the standard at the time of their release.

So, for example, when USB 3.0 was introduced, the number of connector contacts defined by the standard went from 4 to 9. The existing Type A and B connector bodies spawned USB 3.0 versions with essentially the same body but with the additional contacts (although the type B had to add the extra contacts outside the original shell, so the 3.0 type B plug won't fit a 2.0 type B port). Similarly, USB 3.2 is built on top of the existing USB-C connector. It achieves speed increases through protocol changes without requiring any connector modifications.

The standards are backwards compatible. So a USB 3.0 type A connector complies with the 3.0 standard and can deliver 3.0 performance, but it can also be used to make a connection of an earlier standard, like 2.0.

USB-C actually isn't even part of the USB 3.1 release. It is covered by a separate standard that was released roughly the same time as 3.1, and the connector complies with the 3.1 standard. But the 3.1 standard does not require the C connector and they are not synonymous. The USB-C standard actually covers more than just the connector shape and reversibility. It is designed to facilitate a range of uses and features.

  • Thank you very much for your answer fixer123! Your answer definitely clears my concerns about those type letters and version numbers and give me a right angle looking at them with the "standards" you mentioned in the answer, and while they define different things, their releases are closely related to each other with usually the introduction of new standards. I will take a look at the wiki page and come back to digest your answer more deeply👍 – Hang Nov 27 '17 at 11:28

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