In section 2.1.6 we mention a data structure used to represent infinite objects called a stream, which is central to many approaches to exact real arithmetic. Streams are infinite sequences, and are explored in more detail in section 3.2. Importantly, we will see that working with streams is fundamentally different from working with finite data structures like the mantissa of a floating point number. When using a data structure of finite length, it is possible to examine any element of the structure at any point. When using a stream, however, only a finite number of elements at the head of the stream may be accessed at a given time.