MX connects nouns to post-nominal noun modifiers surrounded by commas.

             +----MX--+
             |  +-Xd--+-----Xc------+
             |  |     |             | 
        The dog , a poodle          ,     
        The dog ,    who was black  , barked loudly
        The dog ,   with a big nose , barked loudly
        The dog ,   chasing the cat , barked loudly
        The dog ,   annoyed by Fred , barked loudly
        The dog ,   angry at Fred   , barked loudly        

The MX connectors used here have different subscripts
depending on the kind of modifying phrase. Most kinds of comma
modifier phrases correspond to kinds of no-comma modifier
phrases, which use M rather than MX. However, every word that
takes MX- requires a comma on either side of its phrase; such
words therefore carry (Xd- & Xc+ & MX-). See "X: Comma phrases".

Nouns may take any number of comma modifiers. Thus they carry
"@MX+".

     +--------------MX------------------+
     +--------MX-------+                |
     +----MX---+       |                |
     |  +--Xd--+-Xc+-Xd+-----Xc------+Xd+-----Xc------+
     |  |      |   |   |             |  |             | 
The dog , a poodle , with black hair , who was pretty , ...

Nouns can take without-comma modifiers as well as with-comma
modifiers. However, any comma modifiers must follow any
no-comma modifiers:

        The dog with black hair, a poodle, was pretty
        *The dog, a poodle, with black hair was pretty
        The dog that I bought, a poodle, with black hair, 
              was pretty
        *The dog, a poodle, with black hair, that I bought 
              was pretty 

Nouns therefore have

        dog: {@M+} & {R+ & Bs+ & {[[@M+]]}} & {@MX+}...

             -----------------------------     ----
                without-comma modifiers     with-comma
                                
Nouns can take noun phrases as modifiers with commas. In this
case the commas are obligatory. (There is no corresponding
without-comma phrase; "*The dog a poodle was pretty.") Nouns
therefore have MX-, disjoined with their main "S+ or O-..."
complex:

        dog: Ss- or SIs+ or Os- or Js- or (Xc+ & Xd- & MX-);

Proper nouns can act as noun modifiers too ("My professor,
Ms. Smith, is very good"); thus they also have MX-.
Determiners acting as noun-phrases are given MX- too, although
this is rarely used ("?Jane and Bill, some of my friends, are
here").

Since nouns have MX- as well as MX+; they can act as
noun-modifiers while taking their own noun-modifiers. This is
a frequent source of ambiguity. For example, in the sentence
"My professor, Ms. Smith, an expert on dogs, is here", the
second and third noun-phrases could both modify the first, or
the second could modify the first and the third could modify
the second.

Prepositions and progressive and passive participles also
carry MX-. These usages correspond exactly to Mp, Mv, and Mg,
used for prepositions and participle phrases without
commas. As with Mv and Mg, the MX- on participles must start a
domain; thus it is subscripted MX*p-, and this link-type is
domain-starting.

MX#r: Relative Clauses with Commas
Relative clauses may also act as comma-modifiers. These differ
slightly from ordinary no-comma relative clauses, however.
First of all, the relative pronoun is obligatory, unlike in
no-comma phrases.  Secondly, the relative pronoun must be
"who" or "which"; "that" is not allowed.

        My friend, who you met yesterday, is here
        *My friend, you met yesterday, is here
        *My friend, that you met yesterday, is here

For this reason, we treat comma relative clauses quite
differently from no-comma ones.  The relative pronoun acts as
the head of the phrase. In subject relatives, it makes an
ordinary subject (S) connection to the verb (unlike in
no-comma relatives, where it makes a RS connection). In object
relatives, it makes a B connection to the verb (whereas in
no-comma relatives it makes a C connection to the relative
subject). Therefore it behaves just like a question
phrase. Indeed, the MX- on "who" is directly disjoined with
the W- and QI- connectors used in indirect questions.

who: (B*w+ or S**w+) & (Ws- or Wq- or QI- or (Xd- & Xc+ & MX*r-));

             +--MX--+-S-+
             |      |   |
        My friend, who was drunk, left the party

             +--MX--+----B----+
             |      |         |
        My friend, who Jane hated, left the party

(The subscripts on B+ and S+ are irrelevant to MX-.) One
complication here is that in subject relative clauses, the
verb of the relative clause must agree with the main noun:
"*My friend, who were drunk, left the party."  To handle this,
we use post-processing. MX+ connectors on nouns are
subscripted MXs+ or MXp+ for plural and singular.  This
creates a MXsr or MXpr link with the relative pronoun; these
links are domain-starting.  The verb that the relative pronoun
connects to will then form an "Ss#w" or "Sp#w" link. We then
dictate that a group containing a MXpr must contain a Sp#w;
one containing a MXsr must contain a Ss#w.

Prepositional relatives can also act as comma modifiers.  For
this, prepositions have MX*j-. These connectors act exactly
like Mj-; see "Mj". Finally, adjective phrases can serve as
comma modifiers; adjectives therefore carry MX*a-, which is
similar in function to Ma-.