.CDY file extension

To open .CDY files on Windows, install and open Cinderella, then use its File/Open (or equivalent) to load the .cdy construction file.

To open a .CDY file, use the Cinderella interactive geometry application (the format’s primary creator and editor). If you only need to view the construction, OK Geometry can also browse Cinderella (.cdy) constructions.

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Last updated: June 11, 2026 · Reviewed by Julian Stricker

What “reviewed” means on this page

Format fit: whether the “what is this file?” description matches common real-world use of the extension, including category, typical MIME types, and aliases.
Opening paths: whether Windows / macOS / Linux steps read plausibly for current OS dialogs and default apps; we remove fantasy menus and unsafe shortcuts.
Security framing: whether risk notes match the extension class (for example executables vs plain data) and whether affiliate wording does not contradict the security section.
Sources and further reading: whether external links point to vendors, standards bodies, or other primary references where possible; we avoid inventing details we cannot ground.
Limits: this is clarity and safety-messaging QA, not a guarantee that every statement is exhaustive or that every binary you download is harmless.

Full methodology in the Imprint — The Imprint also states where AI is used and what that does—and does not—replace.

Open on your device

Choose your operating system for a dedicated step-by-step opening guide.

How to open .CDY files

Use these platform-specific instructions to open .CDY files safely.

Windows

Install and open Cinderella, then use its File/Open (or equivalent) to load the .cdy construction file.
If you only need to browse/view the construction, try OK Geometry, which lists support for Cinderella (.cdy) constructions.

Full Windows guide

Mac

Open Cinderella and use File/Open (or equivalent) to load the .cdy file (do not rename the extension).
For viewing/browsing, OK Geometry may be an alternative if you do not need full editing.

Full Mac guide

Linux

If Cinderella is available in your environment, open it and use File/Open to load the .cdy construction file.
Otherwise, transfer the file to a system with Cinderella, or use a compatible viewer such as OK Geometry for browsing if available to you.

Full Linux guide

iOS

There is no widely documented native iOS support for editing .cdy; if Files cannot preview it, move the file to a desktop and open it in Cinderella, or use a compatible viewer such as OK Geometry if your workflow allows.

Full iOS guide

Android

There is no widely documented native Android support for editing .cdy; transfer the file to a desktop to open it in Cinderella, or use a compatible viewer such as OK Geometry if you only need to browse the construction.

Full Android guide

Security notes

.CDY is a specialized geometry construction format; while it is not typically an executable, opening untrusted files still risks vulnerabilities in the parsing application—prefer opening files from known sources in up-to-date software.
Because some tools rely on the extension (and there may be no strong magic signature), verify that a file labeled .cdy actually came from a geometry workflow and is not a different file type disguised with the .cdy suffix.
If a website or email attachment claims to be a Cinderella construction but prompts you to install unrelated software, treat it as suspicious and use the official Cinderella documentation to confirm expected behavior.

If you did not expect this file

This extension is usually plain data, text, or structured content—not a program by itself. The practical risk is social engineering (a scam attachment or misleading filename). For trusted senders you rarely need heavy-handed antivirus wording; use these tools when you want an extra check on unexpected downloads.

Avast

Avast offers free and premium antivirus software that protects against viruses, malware, ransomware, and phishing. Scan files before opening them to ensure safety.

Norton

Norton 360 delivers comprehensive antivirus protection, VPN, and identity theft monitoring. Scan files for threats before opening to keep your device secure.

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Can't open this file?

These are the most common causes and fixes when .CDY files fail to open.

Common reasons

The .CDY file won’t open or opens in the wrong app
The file type is not detected correctly by tools or uploaders
“File is corrupted” or construction does not load fully

Fix steps

Open Cinderella first and use its File/Open command to select the .cdy file.
If you only need viewing, try OK Geometry, which supports browsing Cinderella (.cdy) constructions.

OS-specific troubleshooting

What is a .CDY file?

.CDY is the file extension used for Cinderella “construction files,” which store interactive geometry constructions in a special (application-specific) format. The registered media type associated with these files is application/vnd.cinderella, and filename matching commonly uses *.cdy. Some file identification tools note there is no reliable “magic” signature, so correct file extension and context matter.

Background

Cinderella is an interactive geometry system that saves geometric constructions to disk as “construction files” with the .cdy extension, using a special internal format described in the Cinderella documentation. These files are typically exchanged by teachers and students, or used in math/geometry workflows where interactive constructions (points, lines, circles, dependencies) are saved and reopened later.

On systems that have Cinderella installed, .cdy files are the native way to load and continue working on a construction. In addition to Cinderella itself, some third-party geometry tools can browse or display Cinderella constructions; OK Geometry explicitly lists support for Cinderella (.cdy) among formats it can browse.

For interoperability and detection, the .cdy extension is mapped to the vendor media type application/vnd.cinderella in multiple references (including IANA’s registry and common MIME mapping lists). However, automated file-type detection may be weak because some format databases report no magic signature for this type—so a mislabeled file may not open even if it ends in .cdy.

Common MIME types: application/vnd.cinderella

Common .CDY issues

The .CDY file won’t open or opens in the wrong app

This usually happens when Cinderella is not installed, the file association is missing, or the system doesn’t recognize the format reliably.

Open Cinderella first and use its File/Open command to select the .cdy file.
If you only need viewing, try OK Geometry, which supports browsing Cinderella (.cdy) constructions.

The file type is not detected correctly by tools or uploaders

Some format registries/tools report no magic signature for application/vnd.cinderella, so detection can rely heavily on the .cdy extension and context.

Keep the original .cdy extension; do not rename it to another type to “force” opening.
When uploading or emailing, consider sharing the intended app name (Cinderella) so recipients know what to use.

“File is corrupted” or construction does not load fully

The file may be incomplete (interrupted download/copy) or created by a different workflow/version that your viewer cannot interpret.

Re-copy or re-download the file from the original source and confirm the transfer completed.
Try opening with the original application (Cinderella) rather than a viewer; viewers may support only a subset of features.

FAQ

What program opens a .CDY file?

Cinderella (interactive geometry software) is the primary application for .cdy construction files. OK Geometry also lists support for browsing Cinderella (.cdy) constructions.

Is .CDY the same as a generic “data file”?

In practice, .cdy is specifically associated with Cinderella construction files and the media type application/vnd.cinderella, not a general-purpose data container.

Why can’t my phone preview a .CDY file?

.cdy is a specialized geometry format and is not commonly supported by mobile previewers. Transfer it to a desktop and open it with Cinderella, or use a compatible geometry browser if available to you.

Does .CDY have a reliable file signature for detection?

Some technical references note there is no magic signature for this type, so identification may depend on the .cdy extension and context.

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