Maximal commutative subalgebra

A commutative unital subalgebra $D$ of an associative algebra $A$ is maximal commutative if no larger commutative subalgebra contains it. Equivalently $C_A(D)=D$: any element of its centralizer of a subalgebra generates a commutative algebra together with $D$. The full diagonal matrix algebra inside $M_n(\mathbb{C})$ is an example.