Identifying a cable - help please!

That's a Mini DisplayPort display. It will work plugged into a Thunderbolt 1 or 2 port on a computer, but not with the Thunderbolt port on the back of a Thunderbolt display, or with a Thunderbolt 3 adapter.

machineslikeme Said:

"Identifying a cable - help please!: Hey everyone, Please could I ask your advice on what the middle cable pictured above is? Long story short, I thought I’d bought a Thunderbolt Display, but I thought they only came with two cables on the octopus lead? Thank you so much!"

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That is a Mini Display Port - Go here for adapters Mini DisplayPort

machineslikeme Said:

"Thank you so much! Do you know if there is a way I can check whether my display is the Apple Cinema Display or the Thunderbolt Display? Of course the mini display port suggests that it’s a Cinema Display, but I’d just like to check for sure."

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Do as Instructed Above:

Finding Out More on this Device:

Find the Serial Number* then enter it here** to know what it exactly it is.

Links:

* Find the serial number of your Apple display

** Check Your Service and Support Coverage

machineslikeme Said:

"Identifying a cable - help please!: Also, um, there’s 3 USB ports on the back of the display (and no other ports). I’m starting to think this might be an Apple Cinema Display? Is there any way to check for sure?"

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Finding Out More on this Device:

Find the Serial Number* then enter it here** to know what it exactly it is.

Links:

* Find the serial number of your Apple display

** Check Your Service and Support Coverage

machineslikeme Said:

“Identifying a cable - help please!: Thank you so much! You’ve really helped.”

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You are welcome.

The cable you’re wondering about is the display connection.

The device using a mini DisplayPort will necessarily be the last connection on a Thunderbolt daisy-chain.

The mini DisplayPort and Thunderbolt icons are buried in the text here:

Identify the ports on your Mac - Apple Support