<i>But why complement the bus? The reason is that it&#x27;s easier with CMOS to pull a line low than to pull a line high.</i><p>The other reason, and why some signals are active-low instead of active-high,  is that it saves transistors (and is slightly faster) - due to how gates are constructed, a NAND or NOR is the &quot;natural&quot; construction, and AND or OR gates are actually NAND&#x2F;NOR with an extra inverter on the output. A CMOS NAND-2 or NOR-2 requires 4 transistors, whereas an AND-2 or OR-2 requires 6.