Ordnance Survey
The map products supplied by the Ordnance Survey (OS) play a critical role in the process of Land Registration in Scotland because Part 1, Section 11 (The cadastral map) of the Land Registration etc. (Scotland) Act 2012 states that:
(5) The cadastral map must be based upon the base map.
(6) The base map is -
(a) the Ordnance Map
(b) another system of mapping, being a system which accords with such requirements as the Scottish Ministers may, by order, prescribe, or
(c) a combination of the Ordnance Map and such other system.
On the Ordnance Survey’s website is an interesting section all about the history of the OS from its early days in the 1700s, its military connections, the development of the National Grid, and the evolution from paper to digital maps. https://www.ordnancesurvey.co.uk/about/history
Prior to the introduction of the National Grid, the UK was mapped in the County Series map system, and you can view these historical maps in the Plan Viewer.
If you’d like to know more about the National Grid or how to understand grid references, there are good pages on the Ordnance Survey’s website.
https://getoutside.ordnancesurvey.co.uk/guides/beginners-guide-to-grid-references/?_gl=1*1e346k7*_ga*MTY5MDU5NzIyNi4xNzU2NDgyNTc4*_ga_59ZBN7DVBG*czE3NTY0ODMyNDgkbzEkZzEkdDE3NTY0ODMyNjUkajQzJGwwJGgw*_ga_E5T3PCFCG7*czE3NTY0ODMyNDgkbzEkZzEkdDE3NTY0ODMyNjUkajQzJGwwJGgw - particularly the section called Understanding the National Grid - Eastings & Northings
https://docs.os.uk/more-than-maps/geographic-data-visualisation/guide-to-cartography/coordinate-reference-systems - describes how co-ordinate reference systems work, and has a good section on the National Grid.
https://www.ordnancesurvey.co.uk/mapzone/map-skills/grid-references/page-one - don’t be put off by the fact this is aimed at schools, the content gives a clear and simple explanation of the National Grid and grid references.