A number of different language groups, including Celtic, are classed as Indo-European languages. According to linguists, at some point during the prehistoric period a hypothetical Common Celtic language became divided into two main linguistic groupings: Q-Celtic and P-Celtic. One of the main distinguishing features between Q- and P-Celtic is that one branch, Q-Celtic, preserved a sound which is pronounced [ q u or k w ] (now written c ), whereas P-Celtic speakers changed this sound to [ p ]. One of the best examples of this linguistic division is the word for ‘head’: ceann in Gaelic and penn in Welsh. All of the modern Celtic languages belong to one of these two branches: Irish, Manx and Scottish Gaelic are Q-Celtic languages whereas Welsh, Cornish and Breton are P-Celtic languages.

Although there is a strong tendency nowadays to think of Gaelic as the language of the Highlands and Islands of Scotland, when the Gaelic Notes were written into the Book of Deer c.1130 AD, Gaelic was the common language of the rest of Scotland, including the Lowlands, Ireland and the Isle of Man. Naturally, the form of the Gaelic language changed over many centuries and in different places. One important factor in the history of any language is the introduction of writing. Once a language is written, its written form quickly gets adopted as the only ‘correct’ form by those who can write, and it becomes a standard. In the case of Old Gaelic, the alphabetic system set up c.600 AD remained unchanged for three hundred years. Even if the real language – the spoken Old Gaelic language – changed constantly, the standard written form remained unchanged and became progressively out of date. Perhaps the only surviving evidence of Old Gaelic written in Scotland , possibly c.900 AD, is found in the Book of Deer where the main scribe asks his readers to pray for him.

The phase c.900 to 1200 AD is known as the Middle Gaelic period. A sizeable body of Gaelic literature survives from this time, and the language reflected in this literature has moved away, to some extent, from the strict Old Gaelic standard. This partly reflects a weakening of the control of the óes dána (men of learning) over the language. No new standard was set up c.900 AD, and writers of Middle Gaelic felt free to use innovations of all kinds, as well as retaining archaic conservative forms as they wished. But this freedom, as far as we know, never resulted in any writer accurately writing, or trying to write, the language that was spoken. Consequently, we really know next to nothing of the dialect speech of this period.

It was during this Middle Gaelic period, c.1130 AD, that the Gaelic Notes were inserted into the Book of Deer and they are the earliest considerable body of Gaelic writing known to emanate from Scotland . Doubtless, the scribes spoke the local Gaelic of the North-East, which was almost certainly slightly different from the Gaelic of other areas. Recently, scholars examining the Notes have begun to recognise characteristics which are similar to distinctive features in Modern Scottish Gaelic. This study is still continuing and will have important implications for the history of the divergence of Gaelic into its three component parts, Scottish, Irish and Manx.