Have you ever wanted to display the same note on multiple boards? You can do that by syncing multiple note cards. When you duplicate a note, you get the option to keep the two notes in sync. When you update one note, the other will update too. Only note cards can be synced. It's not possible to sync images, links or to-do items.
Creating a synced note
First, duplicate an existing note card. There are two ways to do this:
Right click the note and choose "Duplicate", or
Hold Alt and drag the note to create a copy (as in the image below)
You'll see the option to 'Keep this copy in sync'. Choose 'Yes'
Viewing other synced notes
Each synced note has a status at the bottom telling you that there are multiple copies of it. The link shows you which boards the other copies belong to.
Removing the connection between notes
Just delete one of the notes to break the connection between them.