Code Systems, Bitting Mathematics & MACS Rules

What a Bitting Code Tells You

Every key cut on a pin-tumbler key has a number assigned to it called a bitting. That number corresponds to a precise depth below the key's shoulder or tip. Manufacturers publish depth-and-space specification sheets that define two things: (1) the exact vertical distance (depth) each cut number represents, and (2) the horizontal distance between cut positions (spacing). Without this data, a key blank is just a piece of metal; with it, a locksmith can originate or duplicate a key entirely from code—no original key required.

Manufacturers divide the total range of cut depth into discrete steps. Kwikset, for example, uses a system where each bitting number from 1 (shallowest) through 6 (deepest) is separated by a fixed increment. Schlage C keyway uses numbers 0 through 9, giving ten depth levels. The total number of theoretical bitting combinations for any key system is:

Total combinations = D^P

where D is the number of depth levels and P is the number of active cut positions on the key.

Depth and Spacing

Each cut position is measured from a reference point—usually the key tip or shoulder—at a fixed spacing interval. Manufacturers specify this in thousandths of an inch (or millimeters in metric systems). When you originate a key from code, you set your code machine or depth gauge to the published depth for each bitting number and cut each position at the correct spacing. Any error in depth or spacing can cause the key to fail to operate or to damage the cylinder.

The key section (also called the key profile or keyway) determines which blank fits which cylinder. Two keys may have identical bittings but different sections and will not interchange. Always confirm the correct blank number from a current code reference before cutting.

The Shear Line and Why It Matters

Inside a pin-tumbler cylinder, spring-loaded driver pins sit above key pins (bottom pins). When no key is inserted, the gap between driver pin and key pin sits below the shear line, blocking rotation. When the correct key is inserted, each cut lifts the stack so that every driver-pin/key-pin boundary aligns exactly at the shear line—the interface between the rotating plug and the stationary shell. The cylinder can then turn. A cut that is too deep lifts the stack too high; too shallow leaves it too low. Either way, the plug cannot rotate.

This is why bitting mathematics must be exact. Even one cut that is off by a single depth increment will prevent operation.

Maximum Adjacent Cut Specification (MACS)

Manufacturers impose a rule called the Maximum Adjacent Cut Specification (MACS). MACS defines the largest allowable difference in bitting depth between any two neighboring cut positions on the same key. The reason is mechanical: if adjacent cuts differ by too many depth levels, the key blade cannot physically guide the driver pins to the shear line—the sharp angle between cuts causes the key to bind or the pins to hang up on the edge of the cut rather than lift cleanly.

To check a bitting for MACS compliance:

1. Write the bitting sequence in order (position 1 through the last position).

2. Subtract each cut number from the number immediately next to it; take the absolute value (ignore negatives).

3. If any adjacent difference exceeds the manufacturer's stated MACS value, that bitting is invalid.

For example, if the MACS is 4 and positions 3 and 4 read `2` and `7`, the difference is 5—a MACS violation. That combination would not appear in a legitimate code book entry and should never be cut.

MACS values differ by manufacturer. Always consult the published specification sheet for the system you are working on rather than assuming a universal value.

Using a Key Code Reference

Published code books such as the Ilco Key Blank Reference or the Kaba Ilco Key Code Manual list key codes organized by manufacturer, keyway, and blank number. For each code you will find:

• The blank number (which blank to use)
• The keyway or key section
• The bitting (the sequence of depth numbers to cut)

Always verify a code lookup against a second source when possible—cross-referencing two editions or databases catches typographical errors that could result in a non-functional key or a wasted blank.

Safety and Professional Responsibility

Key origination from code is a powerful skill that carries legal and ethical responsibility. You must verify customer authorization before originating or duplicating any key. Many states require a locksmith license before performing key origination services commercially—operating without that license is illegal regardless of technical skill. Retain job records including customer identification and authorization as required by your state licensing board.

When operating key-cutting machines: keep guards in place, wear safety glasses (metal filings are a cutting and eye hazard), and keep fingers clear of the cutter wheel. Secure key blanks properly in the machine clamp before cutting begins. If using a code machine with motorized components, follow lockout/tagout procedures during blade changes.