Some tags never travel alone. Tag a city and the country belongs there too. Tag a product SKU and its category should follow. Doing that by hand is easy to forget, and the tag you forget is the one that makes a file hard to find six months later.
Tagalongs handle it for you. Set up a rule once, and whenever a tag gets added to a file, its related tags come along automatically.
Go to Manage Tags and click Tagalongs in the top right, or open Settings → Tagalongs directly.
Each rule has two halves: When this tag is applied, and Also apply these tags. Pick your trigger tag, then add one or more tags that should come with it, and click Add rule.
A few that tend to earn their keep:
If you already keep your tag relationships in a spreadsheet, use Import from CSV rather than adding rules one at a time.
Rules only run while Apply tagging rules automatically is turned on. Leave it off and your rules sit there doing nothing, which is useful while you're still setting them up.
This is the part people don't expect. A Tagalong isn't only for tagging you do from here on - when the rule runs, it also sweeps files that already carry the trigger tag.
So if two thousand files are tagged Paris and none of them say France, creating that one rule fixes all two thousand. Your existing library gets tidied up, not just your next upload.
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Because rules run retroactively, it's worth checking a new rule on a tag with few files before pointing it at your most-used one.
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The win is twofold. You apply one tag instead of three, which is faster. And your library stays consistent, because the second and third tags stop depending on whether someone remembered them. Consistency is what makes filters trustworthy later - see Tagging best practices for more on keeping a library findable.
Tagalongs is available on the Basic, Pro and Enterprise plans.