To begin installing the Unpaywall extension:
Alternatively, click on the relevant icon below:
Get it for Chrome
Get it for Firefox
Next, click on Add to Chrome / Firefox and follow the instructions. You will then see the extension in your browser's menu bar.
A green open lock next to a research article indicates that a full-text version is available when you click it.
A grey closed lock beside a research article indicates that Unpaywall cannot locate a full-text version.