Smart Contract: Storage Layout & Ordering

[gurukamath]

Ordering of storage variables and struct members affect how they can be packed tightly. For example, declaring your storage variables in the order of uint128, uint128, uint256 instead of uint128, uint256, uint128, as the former will only take up two slots of storage whereas the latter will take up three.
https://secureum.substack.com/p/solidity-201