Why do we have to talk about fifty?

Carson a dh'fheumas sinn a bhith a' bruidhinn mu chaogad?

In the conversation just now, we heard Seonag and Murchadh talking about GOC 'rules' and they said that we have to use 'caogad' (fifty) instead of 'leth–cheud' (fifty).

But as we also heard earlier in the conversation, they were just having 'a little fun'!

GOC provides guidance on both systems, as shown here:

GOC provides guidance on Gaelic spelling, which is essential for Gaelic to be taught in schools and used in an official way, but GOC does not deal with dialect or grammar.