Examples from the TeXbook (Chapter 17, more)
TeXbook p. 146
(\; )\; [\; ]\; \{\; \}\; \lfloor\;\rfloor\; \lceil\;\rceil\;
\langle\;\rangle\; /\; \backslash\; |\;\vert\;\Vert\;
\uparrow\; \Uparrow\; \downarrow\;\Downarrow\;
\updownarrow\;\Updownarrow
\Bigg(\; \Bigg)\; \Bigg[\; \Bigg]\; \Bigg\{\; \Bigg\}\;
\Bigg\lfloor\;\Bigg\rfloor\; \Bigg\lceil\;\Bigg\rceil\;
\Bigg\langle\;\Bigg\rangle\; \Bigg/\; \Bigg\backslash\; \Bigg\vert\;\Bigg\Vert\;
\Bigg\uparrow\; \Bigg\Uparrow\; \Bigg\downarrow\;\Bigg\Downarrow\;
\Bigg\updownarrow\;\Bigg\Updownarrow\;
\Bigg\lgroup\;\Bigg\rgroup\; \Bigg\lmoustache\;\Bigg\rmoustache
\bigl(x-s(x)\bigr)\bigl(y-s(y)\bigr)\quad
\bigl[x-s[x]\bigr]\bigl[y-s[y]\bigr]\quad
\bigl| |x| + |y| \bigr|\quad
\bigl\lfloor\sqrt A\bigr\rfloor
TeXbook p. 147
\bigg({\partial\over \partial x^2}+{\partial\over \partial y^2}\bigg)
\big|\phi(x+iy)\big|^2 = 0
\bigl(x\in A(n)\bigm| x\in B(n)\bigr) \quad
{\textstyle \bigcup_n X_n\bigm\| \bigcap_n Y_n} \quad
{a+1\over b}\!\bigg/{c+1\over d}
\bigl(x+f(x)\bigr)\big/\bigl(x-f(x)\bigr)
TeXbook p. 148
1+\left(1\over 1-x^2\right)^3 \quad
\pi(n) = \sum_{k=2}^n \left\lfloor \phi(k)\over k-1 \right\rfloor
TeXbook p. 149
\pi(n) = \sum_{m=2}^n \left\lfloor
\left(\sum_{k=1}^{m-1}\bigl\lfloor(m/k\bigr)\big/\lceil
m/k\rceil\big\rfloor \right)^{-1}
\right\rfloor