The chapter discusses a standard numbering scheme for Class A β-lactamases, enzymes that hydrolyze the β-lactam ring of penicillins, cephalosporins, and related compounds, protecting bacteria from these antibiotics. These enzymes are classified based on their sequences, with the largest group being Class A. The proteins can be found in both Gram-negative and Gram-positive organisms, and they can be cell-bound, periplasmic, or secreted. The chapter highlights the importance of aligning protein sequences to avoid confusion in molecular studies and proposes a standard numbering scheme (ABL) for comparing homologous residues across different Class A enzymes. This scheme is based on aligning 20 Class A sequences and preserving the numbering used by Ambler for the first four members of the class. The ABL numbers will be used to indicate homologous residues, and future corrections and adjustments will not alter the overall numbering. The alignment is reliable but may be refined with X-ray crystal structures, and it is not intended to replace the natural or sequential numbering for individual proteins. The chapter also mentions the network of sequence relations among proteins that interact with β-lactams and suggests that the current scheme can be extended to other classes of β-lactamases.The chapter discusses a standard numbering scheme for Class A β-lactamases, enzymes that hydrolyze the β-lactam ring of penicillins, cephalosporins, and related compounds, protecting bacteria from these antibiotics. These enzymes are classified based on their sequences, with the largest group being Class A. The proteins can be found in both Gram-negative and Gram-positive organisms, and they can be cell-bound, periplasmic, or secreted. The chapter highlights the importance of aligning protein sequences to avoid confusion in molecular studies and proposes a standard numbering scheme (ABL) for comparing homologous residues across different Class A enzymes. This scheme is based on aligning 20 Class A sequences and preserving the numbering used by Ambler for the first four members of the class. The ABL numbers will be used to indicate homologous residues, and future corrections and adjustments will not alter the overall numbering. The alignment is reliable but may be refined with X-ray crystal structures, and it is not intended to replace the natural or sequential numbering for individual proteins. The chapter also mentions the network of sequence relations among proteins that interact with β-lactams and suggests that the current scheme can be extended to other classes of β-lactamases.