Senedd reform: a glossary

Published 06/06/2022   |   Last Updated 20/09/2023   |   Reading Time minutes

The Senedd Cymru (Members and Elections) Bill was introduced on 18 September 2023.

The Bill includes proposals to increase the size of the Senedd from 60 to 96 Members, change the way in which Members are elected and provides for reviews of the boundaries of constituencies for Senedd elections. You can read more about the Bill’s contents here.

This article provide explanations of some of the key terms used in the Bill and in the wider debate around Senedd reform.

A bilingual glossary of terms used in the Bill has also been published.

Key terms

Boundary review

The boundaries for UK election constituencies are usually reviewed periodically to make sure they reflect factors such as population changes. The Committee recommends a streamlined boundary review for the 2026 election, a full boundary review before the 2031 election and reviews periodically after that. These reviews would be carried out by the Local Democracy and Boundary for Wales, which the Bill would rename Democracy and Boundary Commission Cymru. The Bill would also make changes to the Commission itself, including increasing the maximum number of Members of the Commission from five to nine.

Casual Vacancy

A seat which becomes empty between scheduled elections, for example as the result of the death or resignation of a Member.

Closed list proportional representation

A form of electoral system where each party puts forward a list of their candidates for multi-member constituencies. Winning candidates are taken from the lists in order of their position, and seats are allocated proportionally using a formula. In a closed list system, parties put forward a fixed list in an order of their choosing. Voters don’t have a say in how the list is organised, and the voter simply votes for a party’s list. This is the system currently used for regional seats in the Senedd. It is also the system which the Senedd Cymru (Members and Elections) Bill would introduce in a 96 Member Senedd.

Committee on Senedd Electoral Reform (CSER)

CSER was established in September 2019 to consider the findings of the Expert Panel on Electoral Reform. It was made up of Senedd Members from Plaid Cymru, Labour and the Brexit Party (formerly UKIP). The Brexit Party member resigned before CSER published its findings. It published its final report in September 2020.


Co-terminosity is the sharing of boundaries by constituencies for different elections. For example if the UK Parliamentary boundaries and Senedd boundaries were the same they would be co-terminous. Co-terminosity is said to be helpful in terms of familiarity for voters and parties. Senedd boundaries used to be co-terminous with Westminster boundaries until 2011 when the link was severed.

D’Hondt method

A formula used to turn votes into seats in a proportional electoral system. The formula is already used for regional seats in the current Senedd. The number of votes cast for each party is divided by the number of seats the party has already won, plus one. For example, if a party has won two seats, the number of votes won is divided by three. The party with the highest number of votes each round wins the seat, and this is repeated until all seats have been filled. This is the method provided for in the Senedd Cymru (Members and Elections) Bill across a 96 Member Senedd, elected entirely by a closed-list proportional system.

District magnitude

District magnitude is the number of members returned by each constituency. The higher the district magnitude or number of members representing an area the more proportional the results are likely to be. District magnitudes that are too high can risk being ‘hyperproportional’ allowing parties or candidates with very low levels of overall support to win seats.

Electoral Register

The list of people eligible to vote in a particular local area. It is compiled and maintained by Electoral Registration Officers (EROs). There are two versions of the register: the full register, which is used by EROs and returning officers for conducting elections, and the ‘open’ register, which is a version of the full register, and can be sold to any person, organisation or company for a wide range of purposes.

Electoral Terms

The length of time a Parliament sits between elections. The Senedd Cymru (Members and Elections) Bill provides for the length of a Senedd term to decrease from the current fixed five-year term to a fixed four-year term.

Expert Panel on Assembly Electoral Reform

The Expert Panel was established in February 2017 to provide politically impartial advice to the Llywydd and the then Assembly Commission on the number of Members the Senedd should have, the most suitable electoral system to use and other related matters. It consisted of six academics, and was chaired by Professor Laura McAllister. It published its report in November 2017.

Gender quotas

Gender quotas in the context of elections, are rules which demand that a certain number or percentage of a certain gender be represented in the pool of candidates that are up for election.

Job sharing

In this instance, job sharing refers to the joint holding of particular offices, ranging from Member of the Senedd to Presiding Officer to a Welsh Minister. This could be sharing the job concurrently, or one person holding the office temporarily while the appointed or elected holder is unavailable (for example if they were on parental leave). The Senedd Cymru (Members and Elections) Bill would not introduce job sharing provisions, but would place a requirement on the Llywydd to table a motion to establish a Senedd committee to review the issue within six months of the first election held after 6 April 2026.

Legislative Competence

The term used to describe the scope of the Senedd's power to legislate. The ‘yes’ vote in the referendum in March 2011 enabled the Senedd to pass Acts set out in subjects included in Schedule 7 to the Government of Wales Act 2006. The Wales Act 2017 amended the Government of Wales Act 2006 by providing for a new devolution settlement for Wales. The Reserved Powers Model established by the 2017 Act allows the Senedd to legislate on any matters that are not reserved to the UK Parliament.

Mixed Member Proportional System

This is the hybrid voting system currently used to elect Members of the Senedd. It combines elements of first-past-the-post for the 40 constituencies, and proportional representation, where voters select from a list of candidates for each party in five regions, returning a further 20 Members. This helps to overcome the imbalance often associated with first-past-the-post elections. Also known as the Additional Member System (AMS).

Multi-member constituencies

Multi-member constituencies are constituencies which return more than one member. The regional lists used to date in Senedd elections are one example. The Committee proposes in future that each constituency in Senedd elections would elect a total of six members. This is different to ‘first-past-the-post’ electoral systems like the one used to elect UK MPs where just one member is elected to represent a constituency.


The Bill proposes that 16 new Senedd constituencies should be created. It says they should be based on the pairing of the 32 new UK Parliament constituencies that will be in place for the next UK General Election. Pairing means matching together two different constituencies to make one new one. The Bill includes provision for a ‘streamlined’ boundary review to take place to pair constituencies before 2026.

Proportional Representation

An electoral system in which the number of votes cast is closely proportional to the distribution of seats.


The degree to which the distribution of seats within a legislature reflects the share of votes won by parties/candidates. The Expert Panel measured different electoral systems using the Gallagher Index of Disproportionality. This measures the disproportionality of an electoral outcome; that is, the difference between the percentage of votes received and the percentage of seats a party gets in the resulting legislature. The lower the Gallagher figure, the more proportional the outcome.

Sainte-Laguë method

A formula used to turn votes into seats in a proportional electoral system. It’s similar to the D’Hondt method, the main difference being the divider used in each round. In the Sainte-Laguë method the number of votes is divided by twice the number of seats won, plus one. For example after a single seat has been won, the next round uses the divider three, as this is double oner (the seat won in the first round), plus one. This leads to dividers of one, three, five, seven and so on for each round, until all seats have been allocated.

Single Transferable Vote (STV)

A proportional electoral system where voters number candidates in order of preference in multi-member constituencies. Each voter has one vote. If a voter’s first choice has enough votes to win a seat, or if their first choice has no clear chance of winning, the voter’s second choice will be given their vote. Any votes in excess of the quota for the winning candidate will move to the voter’s second choice. This continues until every seat in the constituency is filled. This was the system recommended by the Expert Panel and CSER, but not by the Special Purpose Committee. A minority on the Special Purpose Committee favoured this method.

Special Purpose Committee on Senedd Reform

The Committee set up in October 2021 to consider the conclusions previously reached by the Committee on Senedd Electoral Reform in the Fifth Senedd, and to make recommendations for policy instructions for a Welsh Government Bill on Senedd Reform. The Committee published its report in May 2022.


The Government of Wales Act 2006 says that Bills related to the membership of the Senedd, its constituencies and the systems by which Members are returned must be passed by a supermajority of Senedd Members rather than a simple majority as with other Bills. This means that Bills on these subjects need 40 Members of the Senedd to vote for them at the final stage of the Bill process in order for them to pass. This is set out in detail in Section 111A of the Government of Wales Act 2006.


A procedure where a party’s list of candidates alternates between male and female candidates. This ensures that equal numbers of male and female candidates appear on party lists. This is already used by some parties in Senedd regional elections.

Article by Rhun Davies, Philip Lewis & Nia Moss, updated by Philip Lewis and Josh Hayman, Senedd Research, Welsh Parliament.