A closed string is a one-dimensional fundamental object in string theory that has no end-points, and therefore is topologically equivalent to a circle. Such a closed string can execute one of infinitely many vibrational patterns. One of them always identifies the string with the graviton.

String theory as a whole has not yet made falsifiable predictions that would allow it to be experimentally tested, though various planned observations and experiments could confirm some essential aspects of the theory, such as supersymmetry and extra dimensions.

String theory was originally invented and explored, during the late 1960s and early 1970s, to explain some peculiarities of the behavior of hadrons (subatomic particles such as the proton and neutron which experience the strong nuclear force).

Note that the process of a string splitting (or strings connecting) is a global process of the worldsheet, not a local one: locally, the worldsheet looks the same everywhere and it is not possible to determine a single point on the worldsheet where the splitting occurs.

In a similar manner, in string theory, the elementary particles we observe in particle accelerators could be thought of as the "musical notes" or excitation modes of elementary strings.

The string tension in string theory is denoted by the quantity 1/(2 p a'), where a' is pronounced "alpha prime"and is equal to the square of the string length scale.

String theories are classified according to whether or not the strings are required to be closed loops, and whether or not the particle spectrum includes fermions.

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