Which word is the subject of the clause?

[piggy386]

The work of mathematician Roger Penrose in the early 1970s, on the geometry of what are called aperiodic tiles, turned out to describe the architecture of a previously unknown class of crystals.

Is "aperiodic tiles" the subject of the clause or the complement of "What"?
Please advise.
Thanks.

[Casiopea]

Active
Mathematicians call X aperiodic tiles.
=> Structurally, if 'X' is the object, then 'aperiodic tiles' is its object complement.
Test: X = aperiodic tiles

Passive
X are called aperiodic tiles by mathematicians.
=> Structurally, if 'X' is the subject, then 'aperiodic tiles' is its subject complement.
=> Semantics: if 'X' is the semantic object, then 'aperiodic tiles' is its object complement.

[piggy386]

Originally Posted by Casiopea

Passive
X are called aperiodic tiles by mathematicians.
=> Structurally, if 'X' is the subject, then 'aperiodic tiles' is its subject complement.
=> Semantics: if 'X' is the semantic object, then 'aperiodic tiles' is its object complement.

Thanks, Casiopea.

According to your explanation and the usage notes on http://www.bartleby.com/61/93/W0109300.html, I understand now that Aperiodic tiles is the complement. And when "what" is the subject of a clause and the complement of the main clause are plural, the verb agrees with them