The composition of Parliament was standardized based on the form of the so-call "Model Parliament" of 1295, summoned by King Edward I. Up until that point, Kings had summoned Parliaments on an ad hoc basis, calling whichever nobles, church leaders, and prominant commoners the felt the need to consult.

The Model Parliament consisted of the famiar two houses: the Lords (made up of all adult nobles of rank Baron or above, and all Bishops) and the Commons (elected representatives of commoners who met certain qualifications to vote).

There was no particular effort to apportion constituencies by population. Instead, constituancies were defined to represent specific local communities. Each recognized city (or "Borough") would elect 2-4 "Burgesses" to the House of Commons, and the rural areas of each County would elect two "Knights of the Shire".

The defined communities entitled to representations were not regularly updated to track the rise of new cities and the decline of old cities. The only major modifications I'm aware of before the Great Reform Acts were the addition of University Constitencies by James VI and I in 1603 (adding districts to represent the Universities of Oxford and Cambridges), the merging of the English and Scottish and later Irish parliaments into the British Parliament, and Cromwell's constitutional reforms during the Commonwealth/Protectorate period (which were reversed during the following Restoration period).

Rotten Boroughs and Pocket Boroughs were two distinct but related concepts from the later pre-reform period. A Rotten Borough, strictly speaking, was an urban district which had "rotted" away over the centuries and was now a small and insignificant village. One such borough was Dunwich, which had been a thriving port town in 1295, but shifting coastlines had flooded most of the town and turned most of the rest of it into a swamp by the late 17th century.

A pocket borough was a district (often a rotten borough) with very few eligible voters (voting rights were tied to land ownership in the district), most of whom were beholden to a particular rich or powerful person and would reliably vote as he directed (there was also no secret ballot at the time). This person was said to hold the district in his pocket, as he effectively chose whom the district would send to Parliament.

The US Senate gives a modern, if less extreme, example of the Rotten Borough dynamic. Like the pre-reform Commons, the Senate represents communities with little regard to population, and the communities in question have in many cases become radically bigger (e.g California) or smaller (e.g. Wyoming) compared to the population as a whole. But the system persists, because it's entrenched in law and tradition, because it's baked into existing political power structures, and because many people see value in tieing representation to historical communities rather than strictly to population.

In the 17th and 18th centuries, rotten and pocket boroughs were mostly just a part of the political landscape. The political parties that emerged over the course of that era were largely patronage and favor-trading networks, and courting the support of pocket boroughs was a natural fit for them. Towards the tail end of the 18th century, Whigs started increasingly complaining of the problem of rotten boroughs. Both for ideological reasons (the Whigs generally ideologically favored what we now think of as Enlightenment values) and political ones (the Tory base was wealthy landowners, who tended to be the ones who controlled the pocket boroughs, while the Whig base was urban merchants, often in underrepesented newer town).

It was in the early 19th century that movements to reform rotten and pocket boroughs really got rolling. There were two big reasons for this, both tied to the Industrial Revolution. One was that industrialization was driving bigger and faster shifts in population, including the rise on new major cities which didn't have borough representation at all. The other was that the economy was moving away from land ownership, so non-landowners felt more aggrieved at their lack of political voice. And the example of the French Revolution posed a powerful argument for politicians to take heed of the preferences of the non-voting masses.

Thank you so much, this has helped me in tremendous value. I attribute this by throwing in an opinionated comment of my own, the sole idea of structuring the electoral system requirements off of purely property and/or land ownership perplexes me. Was this parliamentary power attempting to push the industrial working class further and further away from their proverbial cake slice in democracy? As I presume, as you’ve prefaced, that Barons/the hierarchy of the industrial ‘feudal system’ were generally the ones who tended to undertake full ownership of households and land? And I realise a potential reason for this would be the construction and mass expansion of Trade Unionism in the working class population of Britain at the time, who already, on own account, without the representation of parliamentary power, strived for better working conditions and improved economical functions (wages); and therefore, parties such as the conservatives, already perceived that the working class had a voice and therefore needed none in actual democracy, am I right in this? Please forgive me if I am ever so off course from the truth. Thanks once more for your kind and insightful input.