String theories replace the zero dimensional points by one-dimensional strings. It appears impossible to make a consistent quantum theory of gravity based on point particles. Every known consistent one-dimensional classical string theory gives a single massless spin 2 graviton when quantized. The string theory graviton automatically obeys general covariance which is a necessary condition for the classical theory of general relativity of the gravitational field.
Strings can be open with two ends or closed with no ends. Each classical string solution leads to a spectrum of quantized string excitations that are interpreted as elementary particles of matter. The excitations are vibrational, rotational and internal. The internal excitations are described by Lie group and supersymmetries. Type I superstring models have both open and closed unoriented strings. "Unoriented" means the topology of a Mobius strip in which a vector points the opposite direction after parallel transport once around the strip. Type II strings and heterotic strings are only closed and oriented. Each of these models has no ad-hoc adjustable fudge factor dimensionless parameters in the equations unlike the standard model of spinor source fermion quarks, leptons and vector force bosons. However, superstring models allow many different vacuum solutions of these equations. Fudge factors enter at this stage. Unfortunately, these models require a 10 dimensional spacetime when real spacetime only has 4 dimensions.