> ## Documentation Index
> Fetch the complete documentation index at: https://docs.starknet.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Basic Building Blocks

This section covers the basic building blocks used to build the Cairo AIR.

## Felt252 to M31

Cairo works over the prime field $P = 2^{251} + 17 \cdot 2^{192} + 1$, while S-two works over the prime field $M31 = 2^{31} - 1$. Thus, in order to represent the execution of Cairo with S-two, we need to decompose the 252-bit integers into 31-bit integers. The Cairo AIR chooses to use the 9-bit decomposition, so a single 252-bit integer will result in 28 9-bit limbs.

## Range checks

Range-checks are very commonly used in the Cairo AIR. They are used to ensure that the witness values are within a certain range, most commonly within a certain bit length. For example, in the previous section, we saw that a 252-bit integer is decomposed into 28 9-bit limbs, so we need to verify that each limb is in the range $0 \leq \text{limb} < 2^{9}$.

This is done by using a preprocessed column that contains the entire range of possible values for the bit length. For example, for a 9-bit range check, the column will contain the values from 0 to $2^9 - 1$. We also have another column that contains the number of times the range-check was invoked for each valid value and we use lookups to check that each range-check is valid. For a more practical example, please refer to the [Static Lookups](../../air-development/static-lookups/index) section.