Brackets: \[E = mc^2\] Brackets tagged: \[E = mc^2\tag{x}\]
Multline:
\[\begin{multline} a+b+c+d+e+f+g\\ M+N+O+P+Q\\ R+S+T\\ u+v+w+x+y+z \end{multline}\]Multline*:
\[\begin{multline*} a+b+c+d+e+f+g\\ M+N+O+P+Q\\ R+S+T\\ u+v+w+x+y+z \end{multline*}\]Gather:
\[\begin{gather} a_1=b_1+c_1\\ a_2=b_2+c_2-d_2+e_2 \end{gather}\]Gather*:
\[\begin{gather*} a_1=b_1+c_1\\ a_2=b_2+c_2-d_2+e_2 \end{gather*}\]Align:
\[\begin{align} a_1& =b_1+c_1\\ a_2& =b_2+c_2-d_2+e_2 \end{align}\]Align*:
\[\begin{align*} a_1& =b_1+c_1\\ a_2& =b_2+c_2-d_2+e_2 \end{align*}\]Align:
\[\begin{align} a_{11}& =b_{11}& a_{12}& =b_{12}\\ a_{21}& =b_{21}& a_{22}& =b_{22}+c_{22} \end{align}\]Align with and :
\[\begin{align} a_{11}& =b_{11}& a_{12}& =b_{12}\notag\\ a_{21}& =b_{21}& a_{22}& =b_{22}+c_{22} \tag{y} \end{align}\]Align* with :
\[\begin{align*} a_1& =b_1+c_1\tag{z}\\ a_2& =b_2+c_2-d_2+e_2 \end{align*}\]