citing-once

Citing Once

Cite every source once. Gather every citation in the notes or findings you cite from, including two citations of one source in the same note or finding, group them into sources by Same source, then write one Sources entry per source.

A citation is one Sources entry and the metadata block under it:

- [Title](url) — tier text — venue
  - key: value

Tier text is a base tier, tier-1, tier-2 or tier-3, optionally followed by a parenthesised suffix. A citation's tiers are its base tier and every tier its also suffix lists: tier-3 (also tier-1) carries tier-3 and tier-1. Wherever this rule orders or compares citations by tier, it uses the base tier and ignores the suffix. Tiers rank tier-3 lowest, then tier-2, then tier-1.

Same source

Two citations are the same source when they carry the same DOI (see Carrying a DOI) or the same URL after normalisation. Matches chain: citations linked by any sequence of DOI or URL matches are one source. Judge identity only over the citations you read: a merged entry keeps one URL, so a URL an earlier merge dropped links nothing.

To normalise a URL, drop its scheme, drop its fragment, lowercase its host, drop a leading www. from the host, and drop the path's trailing slash. Keep the path and query otherwise unchanged, case included.

URL A	URL B	Sources
https://WWW.Brandur.org/notifier#x	brandur.org/notifier/	one
http://example.org/a	https://example.org/a	one
example.org/a/?id=1	example.org/a?id=1	one
example.org/a?id=1	example.org/a?id=2	two
example.org/A	example.org/a	two

Normalisation decides identity only; it never rewrites a cited URL.

Carrying a DOI

A citation carries a DOI through the doi key of its block, or when its normalised URL's host is doi.org or dx.doi.org: that URL's path, without its leading slash and percent-decoded, is the DOI. The query and fragment are never part of it. Compare DOIs case-insensitively, so https://dx.doi.org/10.X/Y and doi.org/10.x/y carry one DOI, as do doi.org/10.1016/S0140-6736%2820%2930183-5 and doi: 10.1016/s0140-6736(20)30183-5.

Merge rule

Cite a source at its lowest tier across every tier its citations carry. Add each other distinct tier once, next-lowest first, in an also suffix: tier-3 (also tier-2), tier-3 (also tier-2, tier-1). Omit the also suffix when the source has no other tier, so two tier-2 citations merge to tier-2.

Retraction suffixes

A retracted or withdrawn mark on any citation of a source is a property of the source, and the merged tier text keeps it. The suffix lists its parts in order, retracted, then withdrawn, then the also list, each part present only when it applies, joined by ; : tier-3 (retracted), tier-3 (retracted; also tier-1), tier-3 (retracted; withdrawn; also tier-1). A retracted or withdrawn source is always tiered tier-3, so a mark never changes the chosen tier.

Merge rule and Retraction suffixes are the only changes to tier text. Every tier in the merged tier text comes from a citation. Metadata merge, Full ties and Block order are the only changes to a metadata block.

Citation order

Order a source's citations by tier, lowest first, then by venue, then by URL.

• Venue. A named venue sorts before a bare domain. Within each group, venues sort case-folded, then byte-wise. A citation with no venue sorts after every venue.
  • A bare domain is two or more dot-separated labels, each made only of letters of either case, digits and hyphens: example.org, b.example.org, arXiv.org.
  • Every other venue is a named venue, including one with spaces (J. Chem. Phys.) and one word (GitHub, arXiv).
  • So Example Journal precedes example.org, a.example.org precedes b.example.org, acta Journal precedes Zeta Journal, and Example Journal precedes example journal.
• URL. https:// before http:// before no scheme, then byte-wise on the URL as recorded. So https://brandur.org/notifier precedes brandur.org/notifier/, and https://brandur.org/notifier precedes https://www.brandur.org/notifier/.

Representative citation

The first citation in citation order is the representative citation. The merged entry takes its title, venue and URL exactly as recorded, except where Full ties decides.

Metadata merge

The merged block carries every key any citation of the source carries. Each key takes its value from the first citation in citation order that carries it, except where Full ties decides. Never add a key that no citation carries.

Full ties

Citations are fully tied when their base tier, venue and URL are identical as recorded, byte for byte. When the representative citation is fully tied with others, take the title from among them as the first in case-folded, then byte-wise order. When a key's first carrier is fully tied with other carriers of that key, take its value from among them the same way. So Notifier wins over notifier, and accessed: 2026-10-01 over accessed: 2026-10-02.

Block order

List every block you write, merged or carried from a single citation, in canonical order: the recommended keys in the table's order below, then any other key in byte-wise order. Copy each value exactly as recorded, never put into canonical form: no change of case, format or prefix, so doi: 10.X/Y stays doi: 10.X/Y.

Key
doi
arxiv_id
openalex_id
pmid
issn
published
updated
accessed
type
authors
publisher
volume
issue
pages
license
journal_ref
primary_category
oa_status
pdf_url

Worked example

Three citations of one source:

- [The Notifier Pattern](https://blog.example/notifier/) — tier-2 — blog.example
  - accessed: 2026-10-02
- [Notifier](http://blog.example/notifier) — tier-3 (retracted) — Example Blog
  - published: 2019
- [Notifier](https://www.blog.example/notifier) — tier-1 — blog.example
  - openalex_id: W1
  - doi: 10.example/notifier

Citation order puts the tier-3 citation first, then tier-2, then tier-1. The merged entry is:

- [Notifier](http://blog.example/notifier) — tier-3 (retracted; also tier-2, tier-1) — Example Blog
  - doi: 10.example/notifier
  - openalex_id: W1
  - published: 2019
  - accessed: 2026-10-02