Interchangeable parts are parts (components)
that are, for practical purposes, identical.
They are made to specifications that ensure
that they are so nearly identical that they will
            fit into any assembly of the same type.
One such part can freely replace another,
without any custom fitting, such as filing.
This             interchangeability allows easy assembly
of new devices, and easier repair of existing devices,
            while minimizing both the time and skill
            required of the person
doing the assembly or repair.

The concept of interchangeability
was crucial to the introduction of the assembly
line at the beginning of the 20th century,
and has become an important element
of some modern manufacturing
but is  missing from other important industries.

Interchangeability of parts was achieved
   by combining a number of innovations
and improvements (          ) in machining operations
and the invention of several machine tools,
such as the slide rest lathe, *****-cutting lathe,
turret lathe, milling machine and metal planer.
Additional innovations included jigs
for guiding the machine tools, fixtures
for holding the work piece in the proper position,
and blocks and gauges to check the accuracy
of the finished parts. Electrification allowed
individual machine tools to be powered
                                  by electric motors,
eliminating line shaft drives from steam engines
or water power and allowing higher speeds,
making modern large (  ) scale manufacturing possible.
Modern machine tools often have numerical control
(NC) which evolved into CNC (computerized
numeric control) when microprocessors
                                             became available.
Evidence of the use of interchangeable parts
can be traced back over two thousand years
to Carthage in the First Punic War.                                 Carthaginian
ships had standardized,                                      interchangeabl­e parts
that even came with assembly instructions akin
               to "tab a into slot b" marked on them.
In the late 18th century, French General Jean-Baptiste Vaquette de Gribeauval promoted standardized
weapons in what became known
as the Système Gribeauval after it was issued        
                                  as a royal order in 1765.
(Its focus at the time was artillery more than muskets
or handguns.) One of the accomplishments
of the system was that solid cast cannons
were bored to precise tolerances,  
            which allowed the walls
to be thinner than cannons poured with hollow cores.
However,           because cores were often off center,
the wall thickness determined the size of the bore.
Standardized boring allowed cannons to be shorter
                              without sacrificing
accuracy and range because of the tighter fit
        of the shells.                It also allowed standardization of the shells.

Before the 18th century, devices such as guns
were made one at a time by gunsmiths
                              in a unique manner.
If one single component
                             of a firearm                              needed a replacement,
the entire firearm either had
to be sent to an expert gunsmith for custom repairs,
or discarded and replaced by another firearm.
During the 18th and early 19th centuries,
the idea of replacing                                       these methods with a system
of interchangeable manufacture
was gradually developed.
The development took decades                         and involved many people.

      Gribeauval provided patronage
to Honoré Blanc,  who attempted to implement
     the Système Gribeauval at the musket level.
    By around 1778, Honoré Blanc began producing
                        some of the first firearms
                        with interchangeable flint locks,
                        although they were carefully
made by craftsmen. Blanc demonstrated in front
of a committee of scientists that his muskets
           could be fitted with flint locks
                    picked at random from a pile of parts.

                   Muskets with interchangeable locks
caught the attention of Thomas Jefferson through the efforts of Honoré Blanc when Jefferson was Ambassador to France in 1785.
Jefferson tried to persuade Blanc to move to America,
                                       but was not successful,
so he wrote to the American Secretary of War with the idea,
and when he returned to the USA
he worked to fund its development.
President George Washington approved of the idea,
and by 1798 a contract was issued to Eli Whitney
for 12,000 muskets built under the new system.

Louis de Tousard, who fled the French Revolution,
joined the U.S. Corp of Artillerists in 1795 and wrote an influential artillerist's manual that stressed the importance of standardization.
In East Asia during the Warring States
period and later the Qin Dynasty,
bronze crossbow triggers and locking mechanisms
were mass-produced and made to be interchangeable.
Methods for industrial production
of interchangeable parts in the United States
were first developed in the nineteenth century.
The term American system of manufacturing
was sometimes applied to them at the time,
in distinction from earlier methods. Within
a few decades such methods were in use in
various countries, so American system is now
a term of historical reference rather than current
                                      industrial nomenclature.