Original Constituency Groups
Year layers are only shown on a given Constituency Chart for those General Election years when changes to the constituencies on the chart came into effect. For example, although there were widespread changes to constituency boundaries in 1945 and 1955, these are not shown as layers in Chart 1 (Westminster and City of London), as the constituencies in Chart 1 remained unchanged in those years.
Multi-member constituencies are shown with an appropriate width reflecting the number of MPs elected for the constituency. For example, the City of London between 1885 and 1950 in Chart 1 is shown twice the width of the other constituencies, as it was a two member constituency for this period.
Between 1885 and 1918 non-resident freeholders of properties in Borough constituencies were also eligible to vote in a relevant County constituency, so Borough areas could be considered as being in two different constituencies. This is ignored for the purpose of these Constituency Charts and constituency areas are considered mutually exclusive, i.e. in calculating transfers that occurred in 1918, the areas covered by County divisions exclude anywhere that was in a Borough constituency between 1885 and 1918.
Where there are transfers to or from constituencies not included within the main chart, these external constituencies are shown with their chart numbers in italics within a dashed box.
Whether or not a transfer between constituencies is shown depends on the estimate of the number of electors affected (using the methodology described in the Area Maps and Tables section):
No line is shown if the number of electors transferred is estimated to be 2% or less of the 2010 constituency electorate;
A solid line is always shown if the number of electors transferred is estimated to be 9% or more of the 2010 constituency electorate;
If the number of electors transferred is estimated to be between 3% or 4% of the 2010 electorate the line is always shown dashed;
If both the constituencies affected by the transfer have sizes estimated to be more than 50% of the size of the 2010 constituency electorate, then the line is shown dashed if the number transferred is estimated to be between 3% and 8% of the 2010 constituency electorate;
If either of the constituencies affected by the transfer has a size estimated to be less than or equal to 25% of the 2010 constituency electorate, then the line is shown dashed if the number of electors transferred is estimated to be between 3% or 4% of the 2010 electorate, but solid for transfers of 5% or more;
If either of the constituencies affected by the transfer has a size estimated to be less than or equal to 50% of the 2010 constituency electorate (but both are estimated to be more than 25%), then the line is shown dashed for transfers of between 3% and 6%of the 2010 electorate, but solid for transfers of 7% or more.
Irrespective of the above, each constituency at a given time must have at least one solid linking it to another constituency.
The transfer of a complete Borough (or Burgh in Scotland) is also always shown as a line, even if below the 3% threshold.
For example, in Chart 1 (Westminster and City of London), no line is shown for the areas q and r shown on the 1(c) Area Map shown as transferring from Islington S & Finsbury and Bethnal Green & Stepney in 1997, as both of these have estimated size of 1%. The transfer in 2010 from Regent's Park & Kensington N to the Cities of London & Westminster is shown dashed as the number affected is estimated to be just 3% of the 2010 Cities of London & Westminster electorate.
New Constituency Groups
For the new Constituency Charts time is displayed as proceeding from left to right to be consistent with the Area Tables.
Transfers to or from other groups are no longer necessarily shown at the edge of the table, but instead are shown as being to or from an area with a white background. This change was necessary because the 2023 Reviews crossed many long standing boundaries, and keeping the out-of-group transfers going to the edges produced too many unwieldy layouts.
The biggest change has been in the way that the size of transfer is shown. These are now based on the estimated relative percentage of the electorate in a constituency that has come from each of its predecessors. (The original Constituency Charts used the absolute percentages based on 2010 constituencies.) The following conventions are used to denote the size of the transfer:
<3% of the electorate of new constituency: no line is shown;
3% - 10%: a thin dashed black line;
10% - 30%: a thin solid black line;
30% - 65%: a thicker solid black line;
>65%: a thick red line.
A constituency joined by a thick red line to an earlier one is considered its natural successor, and where possible is kept at the same level on the Constituency Chart. For this reason, at most one thick red line is shown as coming from any constituency. If there are two or more constituencies such that more than 65% of each is from the same predecessor seat, a thick red line will be shown emanating from the predecessor only if more than 65% of the predecessor went to a particular constituency.