Table of         String theory details by type and number of spacetime dimensions
Type	Spacetime dimensions	Details
Bosonic	26	Only bosons, no fermions, meaning only forces, no matter, with both open and closed strings; major flaw: a particle with imaginary mass, called the tachyon, representing an instability in the theory.
I	10	Supersymmetry between forces and matter, with both open and closed strings; no tachyon; group symmetry is SO(32)
IIA	10	Supersymmetry between forces and matter, with only closed strings bound to D-branes; no tachyon; massless fermions are non-chiral
IIB	10	Supersymmetry between forces and matter, with only closed strings bound to D-branes; no tachyon; massless fermions are chiral
HO	10	Supersymmetry between forces and matter, with closed strings only; no tachyon; heterotic, meaning right moving and left moving strings differ; group symmetry is SO(32)
HE	10	Supersymmetry between forces and matter, with closed strings only; no tachyon; heterotic; group symmetry is E8×E8