The translation of the 11th sentence of chapter 31 of Assimil "Japanese With Ease" makes me wonder about が and も

Question

あら, この 水色【みずいろ】 の 縁【ふち】が ついた ガウン も 安【やす】い です ね。

So the sentence is part of a dialogue in which two women are shopping. It is translated as:

Oh, this gown with the light blue border is cheap as well.

Since 縁【ふち】 is followed by が , I want to translate it as:

Oh, this light blue border attached to the gown is cheap as well.

Now I do not know about womanly shopping, however, that anyone would buy just the border not the gown seems strange to me. With my limited understanding of Japanese grammar however, I keep translating it as the light blue border attached to the gown being cheap, not necessarily the gown. What type of shop lists the price of the border and the gown separately?

If I look hard and long enough, I can group it in my head as {この} This {水色【みずいろ】 の 縁【ふち】が ついた ガウン} gown to which the light blue border is attached {rest of sentence}, and then も being the "stronger" or "structurally more divisively" particle for the sentence. So が just being part of a subordinate clause and も being part of the main clause. Is that correct? Did I make up the rule of thumb in my head that が marks the more important part of a sentence and that is not applicable here?

Answer 1

あら、この水色の縁がついたガウンも安いですね。

Your analysis is correct. The basic sentence here is:

あら、このガウンも安いですね。
Oh, this gown is also cheap, right?

This is a perfectly formed sentence in its own right.

But you want to be able to describe the gown you're talking about. The thing that does the describing is the relative clause (look this up): 水色の縁がついた. This is also a perfectly formed sentence, "A light blue hem is attached.". But in Japanese you can use a phrase like this in the same way that you would use an adjective, to describe a noun. So 水色の縁がついたガウン is a gown with a light blue hem attached.

In English we use words like 'that/where/which' to describe the relationship e.g 'gown that has a hem', 'gown which has a hem' 'gown where the hem is blue'. These words do not exist in Japanese. Japanese is way easier in this regard.

In a simple sentence with only one clause you might claim that が marks the most important noun, though that is rather over-simplifying, but in a sentence with multiple clauses you can have multiple がs, so they could only ever mark the most important part of their respective clause. I think this が clearly belongs to 水色の縁がついた (or at least it will become clear with a little experience with sentences like this) so there should be no ambiguity.

By the way, it is almost a rule that you must use が and never は in a relative clause, but I believe there are exceptions.