How a STAG works
A STAG is an ad group holding several keywords that mean the same thing to the person searching. Different wordings, one intent. A plumber’s group might carry every way people ask for an emergency call-out, however they phrase it, and nothing at all about boiler servicing.
The test is not how similar the words look but how similar the expectation is. If one set of ads and one landing page could answer every keyword in the group without disappointing anybody, the theme is tight enough. If a keyword would need a different promise, it belongs in a different group.
Why STAGs matter
It solves the problem SKAGs solved without creating the one they created. Ads stay relevant, because everyone in the group wanted the same thing and the landing page can be specific. At the same time the group is big enough to collect conversions, which is what automated bidding and responsive search ads both need before they can improve anything.
Relevance is not cosmetic here. It feeds expected click-through rate and ad relevance, two of the inputs to Quality Score, which in turn affects what you pay and whether you show. A group that mixes intents dilutes both and pays for it in the auction.
Common mistakes with STAGs
The commonest is grouping by topic instead of intent. SEO is a topic; someone searching for what SEO costs and someone searching for what SEO means want entirely different pages, and no single ad can serve them both honestly.
The second is letting a theme drift. Groups start tight, keywords get added because they earned money somewhere else, and a year later the group holds several intents behind an ad that fits none of them. The search terms report is where you notice: when the terms in one group stop resembling each other, the theme has already split.
The third is splitting for its own sake. If two themes would send people to the same page behind the same ad, they are one theme, and separating them only halves the evidence each side has to learn from.
How to act on it
Build from the landing pages you actually have. One page usually equals one theme, and if you cannot name the page a keyword should lead to, you do not yet know which group it belongs in. Write ads that answer the intent rather than parrot the exact phrase, because responsive search ads assemble the headlines for you anyway.
Then review the search terms regularly, add the phrases the group is genuinely matching, and negate the ones showing that a different intent has crept in. When one group’s terms have clearly become two separate conversations, split it — that is the moment a split earns its keep. The same logic shapes how I structure any Google search campaign: separate what needs a different answer, keep together what does not.