It’s 10 hours before the deadline for submitting your summer program application, and you are writing the first of four essays in a Google Docs page when the familiar yellow, or even purple, lines of Grammarly appear below your text. You hover over the text, certain your sentence was fine, but a box appears: “Your tone is ‘uncertain.’”

You read it again, and it seems perfectly fine, so you head over to your iMessages group chat with your friends to rant about Grammarly messing up, when, lo and behold, that same purple line appears, suggesting a way to make your delivery stronger. 

But why is Grammarly there? Why is it active on that screen? It’s the same icon in the bottom right corner of your screen, same writing suggestions, but in a completely different app.

If you’ve ever wondered how this application that made its way into 40 million active users in 2026 manages to wedge itself into almost every native (built for a specific operating system like iOS or Android) and non-native (not tied to any one specific operating system) application you use, then welcome to the Thought Bubble.

Humble Beginnings

Grammarly began in 2009 in Kyiv, Ukraine, founded by Alex Shevchenko, Max Lytvyn, and Dmytro Lider. The trio started off by building MyDropBox, a tool made to detect plagiarism, which was followed by Sentenceworks (now Grammarly) to address the main issue behind plagiarism: students not being able to convey their own thoughts clearly. During its arguably scrappy early days, Grammarly was far from seamless. It was a simple web editor where users had to copy and paste their writing and wait up to 10 minutes for their work to get processed.

What really set Grammarly apart from other startups, both back then and now, is the fact that it used a “backward” monetization model – a model where companies offer only a premium service and no free version. Grammarly was self-funded for almost a decade, and one of the only ways it stayed afloat was by selling premium subscriptions to universities – not individuals like we see today. The company almost went broke many times, until 2011, when they managed to maintain 300,000 paid subscribers, which set the foundation for the Grammarly we see today.

Around 2015, Grammarly shifted to a freemium model and launched a Chrome extension, offering free grammar and spelling checks but requiring a premium model for plagiarism detection and tone adjustments. Since then, the company has exploded from 1 million daily users in 2015 to over 40 million daily users.

How Grammarly Works

Grammarly runs on AI models, allowing it to provide highly accurate suggestions for your writing. These AI models complete various tasks, like grammar checks, spelling checks, and style changes. 

At its core, Grammarly is built on Common Lisp. Common Lisp is known for its efficiency, being able to process thousands of sentences per second – something extremely important for an app as large as Grammarly. 

Essentially, Grammarly uses three steps to break down and analyze text. First, it uses Natural Language Processing (NLP) to break your text into smaller components like words and sentences, allowing it to catch basic errors like subject-verb agreement. Next, it uses a variety of deep-learning and machine learning algorithms to understand the tone, context, and style of your writing. Finally, Grammarly uses reinforcement learning to optimize itself, refining future suggestions based on whether users accept or reject previous suggestions. The thing is, however, all of these steps happen simultaneously, allowing for that instant underline.

Most of Grammarly’s processing occurs on Amazon Web Services (AWS), instead of locally, due to the sheer size of its models, which typically exceed the capabilities of normal consumer hardware. To bridge the gap between your device and the cloud, Grammarly uses Operation Transformation and a Delta Format, which allows for your suggestions to be synchronized and applied to your text almost instantly, even when you continue to write. As a result, most users feel more efficient and confident in their writing.

Entry to applications
But how does it manage to appear across native and nonnative applications alike?

For browsers, that comes down to its content script injection. Content scripts are JavaScript code that, when interacted with via a browser extension, are executed to modify the webpage. When you install the Grammarly browser extension, it gets specific permission from your browser to inject its own content script into any websites you visit. This allows it to “invite itself in” rather than “get invited.”

Once Grammarly has that permission, it is able to read the Document Object Model (DOM), which is the live map of everything on a webpage. It is structured like a tree where every paragraph and text field is a branch.

After creating this tree, Grammarly scans, constantly looking for where you type, and the second you start typing, it “locks on.”

For apps like iMessages, Word, Apple Mail, etc., Grammarly uses a different approach. On macOS, Grammarly uses something called the Accessibility API, which is a system-level tool that is built into macOS. The API allows third-party apps to read and interact with the contents of other apps. On Windows, Grammarly uses the UI Automation (UIA) Framework to read and interact with text fields.

So the next time you see a purple underline appearing beneath your writing, it isn’t just an algorithm correcting your comma splice. Instead, it’s a cloud-based system that’s scanning your syntax, understanding your tone, and slowly learning how you write – all while you’re still typing. On the surface, Grammarly may look like a simple writing tool, but underneath, there’s infrastructure made to integrate itself wherever you type.