Why would a lock require two
keys? Below is a list of some of
the reasons. Each area will be
explored in detail.
1. Expanded master keying, pin,
lever and disk tumbler.
2. Increase security by requiring
two people to be present while
operating a lock.
3. Change the function of a lock.
(Usually used in old padlocks).
4. Create a “Tour Key” system
5. Use as safety interlocks (
US Expanded Pin
Perhaps the largest use of two
keyways is in master keying and
no US Company has used this
concept as much as Yale.
In pin tumbler master keying
if only one keyway (plug) is used,
there has to be two shear lines
for each key cut. This not only
compromises security by doubling the chance of locating the
shear line when picking, it also
greatly limits the total number
of operating key changes in a
For example, if there are 10
possible key depths and because
of a lack of precision in most
US cylinders, there should be a
“safety factor” of 2. This means
the possible number of cuts
in each position is now only 5.
Because an operating key cut
and a master key cut can not be
the same in a split pin system,
the possible cuts have now been
reduced to only 4. The use of a
second keyway (plug) increases
the number of key changes to 5
Therefore, Yale uses its Bicentric cylinders in very large master key systems. The cylinder in
figures 1 & 2 is one of Yale’s first
experimental Bicentric cylinders.
The exact age is unknown, but
the keys it came with are dated
Nov. 2, 1880 and are one of Yale’s
very early key sections. This is
a very large, odd sized cylinder.
Perhaps the size was increased
so that the two cylinders could be
side-by-side. Later Bicentric cylinders staggered the keyways so
that a smaller, standard cylinder
could be made. The gearing allows either plug to turn the cam.
Figures 3 & 4 show two standard cylinders used today. Only
the Yale logos have changed over
Figure 5 shows that the standard size cylinder requires two
Usually the upper keyway is
used for the master key in these
systems and the lower keyway is
for the change key. The Bicentric concept was also used in
padlocks. Two sizes of Bicentric
padlocks were made by Yale
The large lock in figure 6 is
from the Penn Power Co. and the
smaller one is from Ohio State
University. Penn Power once
used these locks all over West-
26 | THE NATIONAL LOCKSMITH | Since 1929 Continued on page 28
1. Early Yale Bicentric Cylinder
2. Rear Yale Bicentric Cylinder
3. Standard Cylinders New Logo
is on Right.
4. Standard Cylinders.
5. Standard Cylinder Note the