One of the largest listings of Tartan designs is "The Scottish Register of Tartan" which is the list maintained by the Govt. of Scotland.

Registration as a member is free and open to anyone. Once a member you can request the thread count of any of the 10,000 - 11,000 designs.

And as stated by Figheadair the thread count will be written in a specific format using an accepted shorthand.

The thread count for the X Marks the Scot Tartan is written -

/W12, DB64, N6, DB6, N2, DB6, N4, A8, DB2, Y4/

With the "/" symbol denoting the pivot points of a half Sett.

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