What was the dominant writing system in Middle-earth?

[Ivriniel]

Thanks for the OP, it's a great read, and reminds me of my first discovery of the Elvish in Middle Earth.

I imagine that Bilbo's language knowledge increased over the course of his life, and from contact with other peoples. So that by the time the Red Book of Westmarch was compiled, the latter authors' contributions seem to reflect learning, and where Frodo and Bilbo must, by then, have been familiar with Quenya.

I don't know why Celebrimbor's speech is referred to as the Mode of Beleriand, in that I wondered how widespread it was. As Sindarin goes, I thought vowels were applied to it as Quenya applied, them, or that a change in Sindarin writing followed from the Noldorin presence. I saw a 'Latin' like comment above, and I tend to agree, and add that I imagine Celebrimbor wrote in Quenya, or with the o and u as his people did. The Ring Spell, with its perversion of Elvish included the 'mirror imaging' of vowels, which seems to give it the creepy sense of it, like playing a record backwards. It must have imprinted dread on Celebrimbor and the Elven Ring wielders to see that.