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newSchoolWorks.com › VoTechWorks › Business & Cross-Cutting › Locksmith & Security Systems › Keying Systems: Master Keying, Rekeying, and Key Control › Lesson 4
BUS-LOCK72-03 · Lesson 4 · 15 clock hours · theory

Master Key System Design and Key Bitting Lists

Objective. The student will design a two-level master key system for a provided scenario, applying MACS rules and producing a complete key bitting list and access matrix free of cross-keying conflicts.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a credentialed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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What Is a Master Key System?

A master key system allows one key—the master key—to open multiple locks, while each lock also has its own individual change key that opens only that lock. This arrangement lets a building owner or manager carry one key for access throughout a property while each tenant or employee carries a key limited to their own space. Understanding how to design these systems correctly is a core competency tested on the ALOA Certified Registered Locksmith (CRL) examination.

The Pin Tumbler Lock and How Bitting Works

In a standard pin tumbler lock, each chamber contains a bottom pin (driver and key pin together) whose total length determines the cut depth required on the key. The key bitting is the sequence of cut depths—one number per chamber—that lifts every pin stack to the shear line, allowing the plug to rotate. In a master-keyed lock, a third element called a master wafer (also called a master pin or spacer) is inserted into one or more chambers. The master wafer creates a second shear line in that chamber, so two different cut depths will both allow rotation—one for the change key, one for the master key.

Key Hierarchy

Most systems use two or three levels:

Some large facilities add a great grand master key (GGMK) above that. For the CRL exam and most vocational scenarios, focus on the two-level (MK + CK) and three-level (GMK + MK + CK) designs.

Maximum Adjacent Cut Specification (MACS)

Every lock manufacturer publishes a Maximum Adjacent Cut Specification (MACS)—the largest allowable difference in cut depth between any two adjacent positions on a key. MACS exists because the key must be able to travel smoothly past each pin without a sharp angle binding the plug or breaking the key blade. If you design a bitting with adjacent cuts that differ by more than the MACS value, the key may be mechanically non-functional or very difficult to operate, even if the math is correct.

Always check every adjacent pair of cuts against the MACS table for the specific keyway you are using before finalizing any bitting. Different manufacturers use different MACS values; a value that passes for one keyway may fail for another.

Designing a Two-Level Master Key System

Step 1 — Gather System Parameters

Collect: the keyway being used, the number of cut positions (typically five or six for common pin tumbler cylinders), the depth increments available (e.g., depths 0–9 or 1–9 depending on manufacturer), the MACS for that keyway, and the number of change keys needed at each level.

Step 2 — Assign the Master Key Bitting

Choose a bitting for the master key. This bitting must itself satisfy the MACS—every pair of adjacent cuts must differ by no more than the MACS value. Write it across a pinning chart.

Step 3 — Develop Change Key Bittings

For each change key, choose a bitting that:

1. Differs from the master key bitting in at least one position.

2. Has no cut in any position that equals the master key cut in that same position (if they matched in every position the change key would simply be a copy of the master key).

3. Satisfies the MACS in its own right.

4. Is not identical to any other change key in the system.

The difference between a change key cut depth and the master key cut depth in the same position is the master wafer size inserted in that chamber. If the difference is zero in a given chamber, no master wafer is needed in that chamber.

Step 4 — Check for Cross-Keying

Cross-keying occurs when a change key unintentionally opens a lock it was not designed to open, because the combination of master wafers in that second lock creates an accidental second shear line alignment with the first change key. To detect cross-keying, compare every change key bitting against every other lock's change key plus its installed master wafers. Any unintended opening is a cross-key conflict and must be resolved by redesigning one of the bittings.

Step 5 — Build the Key Bitting List and Access Matrix

The key bitting list is a table listing every key code and its full bitting sequence. The access matrix is a grid showing which key opens which lock. Both documents are essential deliverables for any master key system job and must be kept in a secure location—they are the blueprint that allows duplication or expansion of the system.

Safety and Security Considerations

Key bitting lists and access matrices are sensitive documents. Unauthorized possession of a bitting list can allow someone to duplicate keys without ever touching a physical key. Store these records in a locked file or encrypted digital storage, and share them only with authorized parties. When designing a system, use key control measures—restricted keyways, patent-protected key blanks, or electronic records—to limit unauthorized duplication. Loss of a master key in a large system may require rekeying the entire system, which is expensive; always document and stress this risk to clients.

Key terms9

Master Key (MK)A key cut to open all locks within a defined group in a master key system, while each lock also has its own individual change key.
Change Key (CK)The individual key that opens one specific lock in a master key system and no other lock in the system.
Master Wafer (Master Pin)A small spacer pin inserted into a pin tumbler chamber to create a second shear line, allowing two different key cuts to both operate the lock.
MACS (Maximum Adjacent Cut Specification)The manufacturer-specified maximum allowable difference in depth between any two adjacent cuts on a key; bittings that exceed MACS may be non-functional or damage the lock.
Key Bitting ListA secure document listing every key code in a master key system alongside its complete cut-depth sequence for each position.
Access MatrixA grid showing which keys in a master key system are authorized to open which locks, used to verify the hierarchy and detect errors.
Cross-KeyingAn unintended condition in which a change key opens a lock it was not designed to open because master wafers in that lock accidentally align with the change key's bitting.
Grand Master Key (GMK)A key at the top level of a three-level system that opens every lock across all groups within the system.
Key ControlPolicies and hardware measures—such as restricted keyways and patented key blanks—that prevent unauthorized duplication of keys in a master key system.

Worked examples4

1. Verify MACS compliance for a five-position Schlage change-key bitting of 3-7-2-6-1, given a MACS of 7.
  1. List every pair of adjacent positions: (3,7), (7,2), (2,6), (6,1).
  2. Calculate the absolute difference for each pair: |3−7|=4, |7−2|=5, |2−6|=4, |6−1|=5.
  3. Compare each difference to MACS 7: 4≤7 ✓, 5≤7 ✓, 4≤7 ✓, 5≤7 ✓.
  4. All pairs pass; the bitting is MACS-compliant.
Answer: Bitting 3-7-2-6-1 passes MACS for a keyway with MACS=7; the largest adjacent difference is 5, which is within the limit.
2. Given a master key bitting of 4-4-4-4-4 (five positions) and a change key bitting of 4-6-4-2-4, determine the master wafer sizes needed in each chamber.
  1. For each position, calculate the absolute difference between the change key cut and the master key cut.
  2. Position 1: |4−4|=0, so no master wafer is needed in chamber 1.
  3. Position 2: |6−4|=2, so a size-2 master wafer is inserted in chamber 2.
  4. Position 3: |4−4|=0, no master wafer in chamber 3.
  5. Position 4: |2−4|=2, so a size-2 master wafer is inserted in chamber 4.
  6. Position 5: |4−4|=0, no master wafer in chamber 5.
Answer: Chambers 2 and 4 each receive a size-2 master wafer; chambers 1, 3, and 5 require no master wafer.
3. A two-change-key system uses master key bitting 3-5-3-5-3. Change key A is 3-3-3-5-3 and change key B is 3-5-3-3-3. Check whether change key A cross-keys into the lock pinned for change key B.
  1. Determine the master wafer in each chamber of lock B by comparing change key B (3-5-3-3-3) to master key (3-5-3-5-3): differences are 0,0,0,2,0. Lock B has a size-2 master wafer only in chamber 4.
  2. Now test change key A (3-3-3-5-3) against lock B's pinning. In each chamber, check if the change key cut equals either the lock B change key cut OR aligns at the extra shear line created by the master wafer.
  3. Chamber 1: CK-A cut=3, lock B CK cut=3, difference=0 — aligns ✓.
  4. Chamber 2: CK-A cut=3, lock B CK cut=5, difference=2; master wafer in chamber 4 not chamber 2 — does CK-A align here? The key pin for lock B chamber 2 is set for cut 5; CK-A presents cut 3, so the shear line is not met — does NOT align ✗.
  5. Because chamber 2 fails to align, change key A does not open lock B. No cross-key conflict exists.
Answer: Change key A does not cross-key into lock B; it fails to align at chamber 2, which contains no master wafer and is pinned for depth 5, not depth 3.
4. You are designing a system with master key bitting 5-3-5-3-5 and need three change keys. You have proposed CK1=5-3-1-3-5, CK2=5-3-5-1-5, and CK3=5-1-5-3-5. Using MACS=7, verify all three change keys are MACS-compliant and are not duplicates of each other.
  1. Check CK1 (5-3-1-3-5) for MACS: |5−3|=2, |3−1|=2, |1−3|=2, |3−5|=2. All ≤7 ✓.
  2. Check CK2 (5-3-5-1-5) for MACS: |5−3|=2, |3−5|=2, |5−1|=4, |1−5|=4. All ≤7 ✓.
  3. Check CK3 (5-1-5-3-5) for MACS: |5−1|=4, |1−5|=4, |5−3|=2, |3−5|=2. All ≤7 ✓.
  4. Compare bitting sequences: CK1=5-3-1-3-5, CK2=5-3-5-1-5, CK3=5-1-5-3-5. No two sequences are identical.
  5. All three change keys are MACS-compliant and unique; none duplicates another.
Answer: CK1, CK2, and CK3 are each MACS-compliant (largest adjacent difference is 4, well within MACS=7) and all three bitting sequences are distinct. The proposed change keys are valid.
In class: Lecture and worked example: instructor walks through the algebra of master keying—master wafers, change-key bittings, top master, grand master—using a whiteboard and a transparency overlay of a pinning chart; students follow along calculating a four-door system together before working independently · Students receive a written scenario (a small office building with a defined access hierarchy: building master, department masters, and individual change keys) and independently produce a key bitting list using a blank master key system worksheet, verifying every bitting pair satisfies the manufacturer's MACS and no two change keys are identical · Peer review: pairs exchange bitting lists, attempt to find MACS violations or duplicate bittings using a MACS checker worksheet, annotate errors, and return for correction; instructor resolves disputed cases
Materials: Blank master key system design worksheets, MACS reference tables for Schlage and Kwikset keyways, sample key bitting lists from ALOA training materials, whiteboard, graph paper, calculator, ALOA Certified Registered Locksmith study guide sections on master keying
Check: Completed key bitting list for the scenario contains no MACS violations and no duplicate change-key bittings; access matrix correctly reflects the stated hierarchy; pass = bitting list reviewed by instructor with zero uncorrected MACS violations and correct matrix, scored on a rubric (pass ≥ 80%)

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Master Keying Principles
1. What is the primary purpose of a master wafer (master pin) inserted into a pin tumbler cylinder?
Key Bitting and MACS
2. A keyway has a MACS of 7. A proposed change-key bitting is 2-9-1-8-2. Which adjacent pair of cuts violates the MACS?
Master Key System Design
3. Cross-keying in a master key system is best described as:
Key Hierarchy and Terminology
4. In a three-level master key hierarchy, which key sits at the highest level and opens every lock in the entire system?
Master Key System Design
5. The master key bitting for a five-position system is 6-4-6-4-6. A change key bitting of 6-4-6-4-6 would be:
Key Bitting and MACS
6. When should a locksmith apply MACS checking during master key system design?
Key Control and Security
7. A client loses a master key in a large master-keyed facility. What is typically the most significant consequence?
Key Control and Security
8. A locksmith is creating a key bitting list for a new master key system. Which document should be stored in a secure, restricted-access location once the system is complete?
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