CTS File

What are CTS files and how to open them

Are you having problems opening a CTS file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your CTS files.

What is a CTS file?

A .CTS file is a TreeSheets Project file.

TreeSheets Project files store structured data created by the TreeSheets application, a tool for organizing information in a hierarchically nested grid system. These files capture mixed content - such as text, lists, calendar entries, mind-map style outlines, and images - arranged inside cells that can themselves contain sub-grids. TreeSheets is the creator and the only known author of files in this format; users build interactive, hierarchical spreadsheet documents in TreeSheets and save them in this native format.

When TreeSheets saves a document, it serializes the entire document tree - including each cell's layout, content, style, and optionally embedded images - into a custom binary format. Images are embedded with deduplication and stored with metadata such as image type (PNG or JPEG), scale, and reference counts. The save data is compressed and version controlled: newer versions of TreeSheets maintain backward compatibility, rejecting files that declare a version newer than the current application supports, while properly parsing files written with older versions.

The header of a TreeSheets Project file begins with a fixed ASCII signature, followed immediately by a version byte and then by image or other metadata before the main content. The exact signature is the byte sequence "TSFF" at the very start of the file (offset 0x0).

How to open CTS files

Important: Different programs may use files with the CTS file extension for different purposes, so unless you are sure which format your CTS file is, you may need to try a few different programs.

While we have not verified the apps ourselves yet, our users have suggested nine different CTS openers which you will find listed below.

Last updated: September 19, 2026

Various apps that use files with this extension

These apps are known to open certain types of CTS files. Remember, different programs may use CTS files for different purposes, so you may need to try out a few of them to be able to open your specific file.

Help us help others

File.org helps thousands of users every day, and we would love to hear from you if you have additional information about CTS file formats, example files, or compatible programs. Please use the links below or email us at submit@file.org to discuss further.