You can order 'slaughter tags', which are single electronic tags, or paired tags (one electronic and one not), depending on what you need.
The numbers are the same, but the individual number on the slaughter tag can only be read by an electronic reader, whereas the individual number on the electronic tag sold as part of a pair is visible, as well as being readable by the electronic reader. (Do not ask me why, it is utter nonsense to me!

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Rules differ by country; I can only speak for England.
As far as I understand it, officially, any sheep that's alive after 12 months old must have two tags.
As far as I understand it, officially, any sheep leaving your holding to go anywhere other than slaughter (via a mart is ok) should have two tags.
In practise, many marts are happy for animals up to 12 months old with a single electronic tag, even if it's a store mart (so the animal will not be going straight to slaughter.)
'Up to 12 months old' is interpreted in a seasonal sense rather than a literal sense - last year's lambs getting sold now are deemed to be 'up to 12 months old', even though some of them may have been born 13-14 months ago.
