How does 3DES maintain backward compatibility with DES?

3DES (Triple DES) maintains backward compatibility with DES by using the same block size and key structure as DES, which allows existing DES algorithms to function without modification. When a 3DES implementation uses a single key (the first key of the three), it effectively operates as DES, ensuring that DES-encrypted data can be decrypted by 3DES systems. This design allows users to transition to 3DES without needing to re-encrypt their existing DES data.