Why is there both 1/24 and 1/25 scale?

This is from the fantastic rec.model.scale FAQ:
[Q] Why are there so many different scales? Where did they all come from?
[A] schmitz@transarc.com (Don Schmitz 8/95)

Lots of different scales are used for models because the real objects
being modeled come in a lot of sizes, while model boxes and display shelves
are all more or less the same size. A scale is chosen so that the model
is a reasonable size regardless of how big the real object was. A number
of standard scales have evolved that manufacturers tend to use for consistency.
Many modelers like to specialize in a particular scale so that the
models in their collections look right when displayed together.

Most modeling scales trace their way back to scales used for
architectural drawings and models. The most commonly used scales tend
to be ratios that make it easy to use standard rulers to do conversions.
For example, 1/12 scale is 1 inch = 1 foot, a scale that works great
for drawing a house floor plan on a desk sized sheet of paper. 1/16
scale is even "nicer", since the 1/16th inch tick marks on a standard
(English) ruler scale out to 1 scale-inch. These two scales (and
their integer submultiples) form the basis of most modeling scales:

1/12 (big scale autos)
1/24 (most autos, *really* big scale planes)
1/48 (big scale planes)
1/72 (common scale for planes)
1/96 (not very common, typically spacecraft)
1/144 (also not very common, spacecraft, airliners)

1/16 (big but not huge autos)
1/32 (another really big scale for planes)
1/64 (S gauge railroad, Matchbox/Hot Wheels sized diecast cars)

Then there are "odd balls" that turn out to be "marriages of convenience"
between odd units or sizes:

1/25th is used by many US model manufacturers as an alternative to 1/24 for
autos. Some cynics feel this is a scheme to save a few cents on the
plastic in each kit, but I think the original motivation was to make
1 mm = 1 inch, so that you could use a standard metric ruler as a
"scale" ruler (for the numerically retentive, that means the scale is
really 1/25.4). An alternative explanation for 1/25th scale is a
scale inch is 0.04 real inches - which is easy to measure using an
engineer's decimal-inch ruler with 0.02 inch tick-marks.

1/43rd, a very common European scale for auto models, is derived from
model railroad practice. 19th century "live steamers" - folks who
build working model steam locomotives and rolling stock - settled on
a nice round 5 inches as the standard gauge (distance between rails)
for their models. The gauge of real railroad tracks is 4'8", so this
scaled out to 1/11.2. As technology allowed model trains to be built
smaller, smaller scales were adopted by the tried and true practice of
using integer sub-multiples. The original 1/11.2 was named "gauge 2",
"gauge 1" was 1/22.4 - still used for big scale (LGB) model railroads,
and "gauge 0" was 1/44.8 (over time "gauge 0" turned into "O gauge").
However someone with too much time on their hands noticed that 1/44.8
was awfully close to the ratio of 7 mm = 1 ft. Why they thought this
was a useful ratio is beyond me - it doesn't allow for any easy
measuring conversion that I can think - but it works out to a scale of
1/43.5. Eventually models trains got to be even smaller, and HO - for
"half O" was born, at a scale of 3.5mm = 1 ft, or 1/87.1.

1/35th, a common scale for military vehicles and armour, has its origins
in figure modeling (lead soldiers). Long before injection molded models
existed, soldier figures were commonly made to a nice round 50mm (2 inches)
tall - which works out to 1/35th assuming a typical 5' 10" human.

Finally, many kits, especially older kits of "odd" subjects are "box scale",
meaning whatever scale allowed them to fill up a standard size model box.
The early AMT Star Trek models tend to fall into this category.

The whole FAQ is at: http://www.ninfinger.org/~sven/models/rms_tips/rmsfaq.1.html

:coolguy:

Way to go reference boy! ;)