There is a nice chat recorded in the last Google webmaster hangout between Mihai Aperghis and with Google's John Mueller. Where they talked about the times you canonical a URL to another to consolidate signals but those URLs have different content on them.
One example was that if you have a page about blue cars and a page about green cars. If you canonical the green car category page to the blue car category page, Google does not know that the green car page has content about green cars since you are telling Google to look at the blue car page and pass the signals there. In short, Google does not see the content on that page.
How does an SEO decide when the page is unique enough to not want to canonicalize it to a different page. That is the big question and something that requires a balancing act, the two said.
Also, what happens when you remove the canonical and the flow of signals stop going to the green car and then go to the blue car.
It is an interesting chat, here is the video at the start time followed by the transcript - this starts at 45:05 in to the video:
Mihai Aperghis: Is when you have a lot of URLs doing something like they all have a canonical to a certain page and then you change it. And all of those URLs no longer have a canonical but they do something else, maybe they're just indexable.
Maybe let's say you have an e-commerce website and you have a category page and all of the filters or something like that they all have a canonical back to the main category page. But then you decide okay I need those filters to be indexed or redirect them somewhere, I don't know, and you break the canonical and then you use them in whatever other way you want to use them.
Does that mean that the main category page starts to lose some of the signals that were until now aggregated from all of those filters and things like that? Is it kind of a balance you kind of gain some you lose some.
John Mueller: Probably. I mean it it depends a little bit on on what you mean with signals. Because with a canonical URL, when you specify that, we don't take the content itself into account. So if you have a blue car in the canonical as a green car, then we will index the green car and we won't know about the blue car. If someone links to the blue car then we know like there's a link that indirectly goes to the green car, but we don't know it's like blue car.
Mihai Aperghis: Right but that means that if you start making the blue car indexable or do it whatever else with it. That means that the blue car will start gaining maybe in in rankings and get traffic especially if it has links and everything else but you're no longer canonicalizing to the green car so the green car kind of loses those ranking signals that came through the canonical until now. So it's yeah you lose some but you might get a lot more traffic by being more specific towards certain queries.
John Mueller: I think that's always a balance that you have to figure out. Like there's no absolute answer where you can say you should always split it up or you should always combine it. Sometimes it makes sense to split sometimes it makes sense to keep it separate.
Forum discussion at YouTube Community.
