How the timeline was made

What is in scope

The timeline begins in 27 BCE, the conventional start of the Roman Principate, and ends in 2026. It includes a comparative selection of monarchies, predecessors needed to explain the displayed transitions, and all twelve sovereign European monarchies at the 2026 snapshot. A polity may be included because it clarifies a transition, a ruler holding several crowns, or a regional comparison. That includes selected emirates, caliphates, duchies, electorates and principalities. Inclusion does not imply modern sovereignty: a crown could survive inside a larger imperial or composite framework.

All included monarchies appear on the timeline. Search jumps to a monarch, dynasty, country or region without hiding the surrounding rows. East Francia, Middle Francia and Ireland have their own rows so their unions and partitions can be followed.

Kyivan Rus’ is treated as shared medieval eastern European history rather than the exclusive predecessor of a single modern state. Galicia–Volhynia records one western Rus’ line of continuity into lands later divided between Poland and Lithuania. Muscovy is shown separately because its direct institutional transition into the Tsardom of Russia in 1547 should not turn the broader Rus’ past into a single Russian state genealogy.

"Europe" is treated as a historical field rather than a hard modern border. The Ottoman Empire is included because its European territory and diplomacy were central to the continent. Byzantium is included even when much of its territory lay in Anatolia. Colonial possessions are not drawn as extra lanes.

One lane does not always mean one state

A lane is an editorial container. Most represent a polity, but some join closely related constitutional phases so the overview stays readable. France, for example, contains West Francia, the pre-revolutionary kingdom, the Napoleonic empires, the Bourbon Restoration and the July Monarchy. The gaps between those bars matter. They show that the monarchy was interrupted.

England, Ireland, Great Britain and the United Kingdom have separate lanes because the Acts of Union created new states. Castile and Aragon remain visible through the composite-monarchy period; the Nueva Planta decrees of 1707–1716 abolished most separate public institutions of the Crown of Aragon and generalized a more Castilian Bourbon framework, but did not erase every regional law in one stroke. Spain also has its own overlapping dynastic phase from 1479. That overlap is deliberate: Isabella and Ferdinand shared a monarchy without erasing the two crowns.

The medieval Italian crown is separate from the modern Kingdom of Italy. The Kingdom of Burgundy / Arles appears as a contextual event; the later Burgundian ducal territories have their own lane. The additions retain selected rulers, reuse people holding several titles, and preserve interruptions instead of filling every gap with a dynasty band.

Why this is not a Sankey diagram

Sankey diagrams are built for directed flows with a measurable quantity. Energy, money and material can be divided and recombined, so the width of a stream has meaning. A crown has no comparable conserved quantity. Making the Habsburg line twice as thick as the Tudor line would look informative without saying what was measured.

Every polity bar therefore has the same height. Width encodes time and nothing else. Connectors record continuity, conquest, dissolution, dynastic union, partition, personal union, composite monarchy, restoration, state union or constitutional reorganization. An interrupted union framework is labelled separately from continuous shared rule. A dotted conquest line is a historical claim; a fat stream would be an unsupported metaphor.

A dendrogram has a different problem. It assumes a hierarchy and ordinarily places leaves at the same depth. That is useful for hierarchical clustering and phylogenetic trees. European dynastic history is a network with marriages, elective crowns, restorations, cadet branches and people who held several crowns at once. Trees remain useful for a focused genealogy, but they are not the main structure here.

What each mark means

The data model

The original prototype stored a monarch directly inside one monarchy. That breaks as soon as the same person holds two crowns. James VI and I should be one person with a Scottish reign and an English reign, not two people who happen to share a name.

The current dataset separates six things:

• polity: the named lane and its editorial summary
• phase: a dated period when that polity or monarchical institution existed
• house and rule: a dynasty plus its dated association with a crown
• person and reign: one human linked to one or more titled reigns
• relationship: a typed connection among polity lanes
• event and source: dated context and the references used to support it

This model also handles Charles V. He has a Spanish record beginning in 1516, when he assumed the crowns jointly with his mother Joanna, and an imperial record beginning with his election as King of the Romans in 1519. Moving the year line between those dates shows the difference; his papal coronation as emperor occurred in 1530.

Dates and disputed periodization

Many beginnings and endings are conventional. The Roman Empire did not legally split into two unrelated states in 395, even though separate courts and successions became enduring. The western imperial tradition revived by Charlemagne in 800 and the continuous line from Otto I in 962 offer different starting points for what historians later call the Holy Roman Empire. France can be traced from West Francia in 843 or from the Capetian accession in 987. "Spain" can refer to the dynastic monarchy of 1479 or to the more uniform Bourbon order built through the Nueva Planta decrees of 1707–1716.

The project records the chosen year and explanatory note, with machine-readable precision flags where added. Missing evidence remains a review item. Relationship lines connect institutions active during the transition year, or a predecessor ending in the preceding year for a January transition; they cannot jump across a missing century of history. The Holy Roman lane starts in 962, while Charlemagne's coronation remains visible as an event in 800.

Years before the Common Era use negative integers in the data. The interface prints them as BCE and never displays a year zero. Continuing phases, house periods and reigns use a null end rather than repeating the publication year; the chart extends them to the snapshot year from the dataset metadata and labels them as ongoing at that snapshot. The slider includes every recorded phase or reign active at some point during the selected year. Thus England, Scotland and Great Britain all appear in 1707; this is annual overlap, not simultaneous existence at a single instant. Dates are not moved to neighbouring years to manufacture visual gaps.

Sicily, Naples and the Two Sicilies are separate lanes: the island remained under Bourbon rule while Napoleon’s appointees ruled Naples. Poland distinguishes royal, ducal and fragmented periods; Croatia continues through its shared crowns to 1918. Kalmar and the early Polish–Lithuanian union are political frameworks with interrupted shared rule, not continuous joint reigns.

Sources and verification

Editorial notes, Wikipedia context and source evidence serve different purposes. Notes explain how a record is represented. A direct citation identifies a source and, where available, the relevant section or page. A polity’s general bibliography is shown as background reading; it is not silently treated as proof of each reign or dynasty band.

The September sourcing pass attached direct references to every dated phase, house period and selected reign, with specialist or institutional support for every event and connection. Citation locators identify a section or table only when it has been checked; qualifications about boundaries belong in record notes. This makes the claims traceable; it does not replace a complete regional specialist review. Inspect the current data-quality report before treating the chart as evidence for a detailed historical claim.

Wikipedia excerpts come from explicitly mapped articles, with redirects resolved before publication. Each excerpt is stored locally with its article, revision and retrieval date. It is shortened to a few sentences and pronunciation-only parentheses are omitted. The excerpt supplements the editorial note; it never replaces it or selects a different article through live search.

Updates, tests and corrections

The canonical file is data/timeline.json. A deterministic build writes a compact data bundle and the data-quality report. The validator checks IDs, required text, references, date bounds, complete coverage within institutional phases, valid transition endpoints, state-union inputs, citation links and Wikipedia provenance. The website itself has no database or server dependency.

The repository’s TASKS.md records the action checklist; IMPROVEMENTS.md preserves editorial scope, review history and limits of the annual-scale model. Regression tests preserve consequential fixes such as the English Interregnum, Monaco's French-annexation gap, Majorian's corrected house assignment and the 1917–1922 sequence of imperial collapse.

Corrections are welcome as focused GitHub issues. A useful report names the record, gives the proposed date or wording, and links to a source. Questions of periodization may result in a better note rather than a single "correct" year.

Report a correction on GitHub

Accessibility and responsive design

All controls are native form elements. Polity labels, phases, house bands, monarch dots and relationship symbols are keyboard-focusable buttons with descriptive labels. Selecting any mark writes the same information into a visible detail panel, so hover is never the only route to the text.

The desktop view uses a single horizontal time axis. Search matches realm, ruler and dynasty names; related-record links make small or hidden marks accessible in text. Year and selected record are stored in the URL. On small screens the chart is transposed: time runs down the page, each monarchy is a column, and the connections are drawn between columns. Motion is reduced when the operating system requests it, and meaning is paired with labels and line styles rather than colour alone.

License

Original data, code and writing are available under the MIT License. Cached Wikipedia excerpts are adaptations by Wikipedia contributors under CC BY-SA 4.0, with article and revision links beside each excerpt. Other linked references retain their publishers’ terms.