  We study two and three-point tree-level amplitudes of open strings. These
amplitudes are reduced from higher-point correlation functions by using mostly
BRST exact operators for gauge fixing. For simplicity, we focus on an open
string tachyon. The two-point amplitude of open string tachyons is reduced from
a three-point function of two tachyon vertex operators and one mostly BRST
exact operator. Similarly the three-point amplitude is from a four-point
function of three tachyon vertex operators and one mostly BRST exact operator.
One can also obtain the two-point amplitude from the four-point function of two
tachyon vertex operators and two mostly BRST exact operators. In these
derivation from four-point functions, moduli integrals are significant. We
discuss the overall signs of amplitudes which are indefinite in this formalism.
