# Complexity Zoo:List of Hierarchies

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*Complexity classes by letter:*
Symbols -
A -
B -
C -
D -
E -
F -
G -
H -
I -
J -
K -
L -
M -
N -
O -
P -
Q -
R -
S -
T -
U -
V -
W -
X -
Y -
Z

*Lists of related classes:*
Communication Complexity -
Hierarchies -
Nonuniform

The concept of a **hierarchy** of complexity classes is common enough that we list here some of the more notable examples of classes that are defined as the unions of hierarchies.

- AH - Arithmetic Hierarchy
- BH - Boolean Hierarchy
- CH - Counting Hierarchy
- EH - Exponential-Time, Linear-Exponent Hierarchy
- FH - Fourier Hierarchy
- PH - Polynomial-Time Hierarchy
- PH
^{cc}- Polynomial Communication Complexity Hierarchy - QCPH - Quantum Classical Polynomial-Time Hierarchy
- QPH - Quantum Polynomial-Time Hierarchy