What Is a Legal Blackline? (And How to Produce One in Word)
A partner sends you an NDA. "Blackline it against round two and let me know what they changed." This is a standard request, and "blackline" is a word you will hear in almost every contract negotiation workflow, but it is rarely defined. Most lawyers pick up what it means by context and then go looking for a how-to the first time they have to produce one themselves.
This post is that how-to. We will define what a legal blackline actually is, walk through how to produce one in Word, cover what Word's built-in tool gets right and where it falls short, and talk about when a standalone comparison tool becomes worth the switch.
What is a legal blackline?
A legal blackline is a comparison between two versions of a document, presented as a single file that shows the differences inline. New text is marked as an insertion — usually underlined, sometimes in a distinct colour. Removed text is marked as a deletion — usually struck through. A reader working through the blackline sees both the old wording and the new wording in place, and can tell at a glance exactly what changed.
The artifact is the same whether it is called a blackline, a redline, or a marked-up version. What makes it "legal" is just that the workflow is ubiquitous in legal practice: almost every contract negotiation involves exchanging blacklines between rounds, and almost every law firm and in-house legal team has a preferred tool and a preferred set of conventions for producing them.
The defining feature of a blackline, versus a simple side-by-side comparison, is that the differences are expressed in context inside a single document. A reviewer does not have to flip between two windows to understand what changed. The blackline is a self-contained artifact you can email, print, and annotate, and the reviewer always sees the change in the middle of the clause it modifies.
Where the term comes from
Before word processors, lawyers marked up contracts in ink. The common convention was to use a black pen for edits (strikethroughs for deletions, inserted text in the margin or between lines), which is where "blackline" originates. "Redline" described the same thing in firms that used red ink, or in contexts where a specific reviewer's marks were distinguished by colour. The two terms describe the same practice and are used interchangeably in modern usage.
Microsoft Word picked up "Legal Blackline" as a specific option in its Compare dialog, which is one reason the term still shows up in day-to-day practice. When you select that option in Word, you get a comparison rendered as tracked changes in a new document — exactly the artifact lawyers have been producing for decades.
For more on the history and when the terminology matters, see our post on redline vs blackline.
How to produce one in Word
The mechanical steps are short:
- Open Microsoft Word (any version from 2010 onward will have this feature).
- Go to the Review tab in the ribbon.
- Click Compare → Compare.
- In the dialog, pick the Original document (the baseline) and the Revised document (the new version).
- Expand the More options if you want to control what gets compared — formatting, case changes, whitespace, etc. Defaults are fine for most cases.
- Click OK.
Word opens a new document containing the blackline. The insertions appear as tracked insertions, the deletions as tracked deletions, and the result is a standard .docx file you can save, email, or further annotate.
One detail that trips new users: Word's Compare is different from Word's Combine. Both live in the same menu. Compare produces a blackline between two finished documents. Combine merges tracked changes from two reviewers into one document. The one you want for a blackline is Compare, not Combine. Our post on combine vs compare vs consolidate covers the distinction in detail.
What Word's Compare gets right
Before criticising the limitations, it is worth being clear about what Word's built-in blackline does well. It is free, it is already on every lawyer's machine, and for straightforward contracts it produces a perfectly usable blackline. The output is a native .docx file with native tracked changes, which means it opens on any system, it can be emailed without compatibility concerns, and it can be annotated, accepted, or rejected with the same tools anyone else uses.
For short agreements — an NDA, a letter of intent, a simple services order — Word's Compare is the right tool. The blackline is readable, the review is fast, and there is no reason to reach for anything more complex.
Where Word's blackline falls short
The limitations appear when the contracts get longer or the edits get more subtle. Three specific failure modes come up repeatedly:
1. Every change shown equally
Word's Compare treats a change from "30 days" to "60 days" the same way it treats a change from two spaces to one space. Both are differences. Both are marked. In a 60-page services agreement with 200 marked changes, the material edits — the ones you actually need to review — are buried in formatting drift and whitespace fixes. Manual triage is required before review can begin.
2. Moved paragraphs render as delete-plus-insert
When the revised document moves a paragraph from one section to another without changing its content, Word's Compare typically renders the move as a full deletion of the paragraph in its old location and a full insertion of the same paragraph in its new location. A careful reviewer has to manually confirm that the two versions are identical. An inattentive reviewer reads the same text twice.
3. No rationale attached to changes
A blackline tells you what changed. It does not tell you why. In multi-round negotiations, the why matters — "they tightened the indemnification cap from $5M to $2M, which is a standard give we can accept if they concede on the IP carve-out." Word leaves you to reconstruct rationale from memory or from a separate negotiation tracker.
When to switch to a standalone tool
Standalone blackline tools — Litera Compare, Draftable, and clausul — exist because the limitations above stop being acceptable at a certain level of contract complexity. The threshold varies, but a few rough heuristics:
- Contract length. Above 20-30 pages, Word's undifferentiated change list becomes hard to triage. A tool that separates material edits from formatting noise pays for itself in review time.
- Round count. Past three or four rounds, small changes accumulate and the eye starts to skim. A tool that surfaces what is new this round without re-marking what was agreed in earlier rounds is worth the switch.
- Stakes. When a missed edit has material consequences — M&A, major financing, critical vendor contracts — the cost of missing a change due to triage fatigue outweighs the cost of a better tool.
- Document complexity. Contracts with tables, defined terms, and extensive cross-references are where Word's character-level comparison produces the most noise. Semantic comparison tools handle these better.
Clausul is one option in this space. We focus on producing blacklines that separate material changes from formatting noise, detect moved clauses, and attach short change notes to each edit so reviewers can skim the substance. See the legal blackline page for what that looks like concretely, or the compare page for the product-level overview.
Frequently asked questions
What is a legal blackline in plain English?
A legal blackline is a comparison between two versions of a contract, presented as a single document where the additions from the new version are marked as insertions and the removals from the old version are marked as deletions. It is how lawyers communicate "here is what changed" between drafting rounds, and it is the standard artifact exchanged in most contract negotiations.
Is there any difference between a blackline and a redline?
In current U.S. legal practice, no. The two terms are used interchangeably. The historical distinction — blackline for ink-on-paper markups, redline for tracked edits — has collapsed over time. Some firms prefer one term over the other, but the artifact is the same. Our post on redline vs blackline covers the terminology in more detail.
How do I make a legal blackline in Word?
Open Word, go to the Review tab, click Compare, and select Compare. Choose your original and revised documents in the dialog, and Word will generate a new document containing the blackline with tracked changes. The result opens as a standard .docx file with insertions and deletions marked inline. You can then save it, accept or reject individual changes, or export it.
Why does a blackline matter more than just reading the new draft?
Because what changed is almost always more important than what stayed the same. A 60-page contract might have twenty material edits buried in otherwise identical text. Reading the new draft cover-to-cover forces your eye to process every word equally, and material edits are easy to miss. A blackline directs attention to the actual changes, which is what you need to review in a negotiation round.
What are the limitations of Word's built-in blackline?
Word's Compare treats every character-level difference equally. Formatting changes, punctuation fixes, and whitespace drift appear alongside material edits to numbers, dates, and obligations. Moved paragraphs are typically rendered as a full deletion followed by a full insertion, so you have to manually confirm the moved text is identical. For short, simple contracts this is fine. For longer or more complex agreements, you end up manually triaging noise before you can start reviewing substance — which is why standalone comparison tools exist.