What are those sidecar files to my QuickBooks Company file

Extension	What the acronym means	Purpose in day-to-day QuickBooks	When it?s read/updated	Can you delete or regenerate it?
.TLG	Transaction Log	A running log of every change written to the company file since the last verified manual backup. QuickBooks uses it to: 1) finish writing any transaction that was interrupted by a crash/network drop, and 2) ?re-hydrate? data when you restore a *.QBB backup (QuickBooks re-plays the TLG onto the restored file to bring it current).	Updated continuously while the file is open; reset automatically when you create a manual backup with verification or restore a portable file.	Keep it until you?ve made a fresh verified backup?then you may archive or delete it to reclaim space. It?s critical for data recovery, so don?t trash it mid-year.
.ND	Network Descriptor	A tiny text file that tells QuickBooks workstations where the QB Database Server is listening (IP, port, server engine name, file path) and whether the file is already open. Without it, workstations can?t find or share the company file in multi-user mode.	Read first every time a workstation tries to open the .QBW; rewritten when the Database Server Manager scans the folder or when hosting settings change.	Safe to delete and let Database Server Manager re-scan the folder?QuickBooks will regenerate a clean copy. A damaged or duplicated .ND is the root cause of many H101/H202/H505 errors.
.DSN	Data Source Name	Stores ODBC connection parameters (driver, server, credentials, application ID/Name) that let external programs?Excel live reports, QODBC, custom middleware?talk to the QuickBooks database through the ODBC driver. QuickBooks itself doesn?t need it for core operations; it?s only invoked when an add-on or report uses ODBC.	Read whenever an external app initiates an ODBC session; not touched during normal QuickBooks use.	You can recreate or edit it from Control Panel ? ODBC Data Sources (File DSN tab). If the file is missing or points to the wrong path, ODBC integrations will fail, but QuickBooks Desktop will open normally.

Practical take-aways

Backups: Always include the .TLG with the .QBW in your backup set; Intuit?s Data Protect and most image-based backups already do this. The .ND and .DSN files are optional because QuickBooks (or Windows ODBC) can regenerate them, but keeping them speeds up recovery.
Troubleshooting tips:
- Massive .TLG (> 1× the size of the company file) usually means you haven?t run a verified manual backup in a while. Make one to roll the log data in.
- Delete/rename and re-scan the folder with Database Server Manager any time users get H-series network errors; that forces a new .ND.
- If an ODBC report suddenly fails, open the .DSN in Notepad?verify the path still matches the company file and the driver version hasn?t changed.

These three small files act as QuickBooks? safety net (TLG), traffic cop (ND), and external gateway (DSN); understanding them makes both backup strategy and troubleshooting far smoother.

How big should a TLG file be to require a Data Verify?

A QuickBooks TLG (Transaction Log) file typically requires attention?or a Data Verification?based on activity volume, not just file size, but here are the practical thresholds:

Rule of Thumb:

If your .TLG file is larger than 20% × the size of your .QBW file, you should:

Run a Verify (DO NOT run a Rebuild, it will jank-up your file).
Consider creating a portable company file
Consider archiving / truncating the file (see below)

Safe TLG Size by Use Case

Company Size	QBW Size	TLG Size Threshold	Action
Small (1?5 users)	< 500 MB	> 150 MB	Run Verify
Medium (5?15 users)	500 MB ? 2 GB	> 300 MB	Run Verify, then Backup with Full Verify
Large (multi-user, high activity)	> 2-5 GB	> 1 GB	Run Verify, plan periodic archive or TLG reset