In this chapter, we've already dealt with the concept of superposition of single qubits: two or more qubits combined to produce another qubit.
Some of these examples of superposition are when we wrote |"+">, |"-">, 
or 
in terms of |"0"> and |"1">. None of |"+">, |"-">, or are 100% |"0"> or 100% |"1">; they are all a mixture of each. |"0"> and |"1"> can also be written as a combination of other qubits. We can write both as a combination of |"+">, |"-"> or a combination of and , for example:
And for |"1"> we can write these combinations as follows:
Two or more qubits can always be added together to produce a valid qubit. Any qubit can be represented as the sum of two or more other qubits. This is the principle of quantum superposition for qubits.
In the next chapter, we'll see examples of superposition acting on multiple qubits, known as quantum states, at once.