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The slope and zero-point of the unevolved main sequence as a function ofmetallicity are investigated using a homogeneous catalog of nearby field starswith absolute magnitudes defined with revised Hipparcos parallaxes, Tycho-2photometry, and precise metallicities from high-dispersion spectroscopy.(B-V)-temperature relations are derived from 1746 stars between [Fe/H] = -0.5and +0.6 and 372 stars within 0.05 dex of solar abundance; for T_e = 5770 K,the solar color is B-V= 0.652 +/- 0.002 (s.e.m.). From over 500 cool dwarfsbetween [Fe/H] = -0.5 and +0.5, Delta(B-V)/Delta[Fe/H] at fixed M_V = 0.213 +/-0.005, with a very weak dependence upon the adopted main sequence slope withB-V at a given [Fe/H]. At Hyades metallicity this translates into DeltaM_V/Delta[Fe/H] at fixed B-V = 0.98 +/- 0.02, midway between the range ofvalues empirically derived from smaller and/or less homogeneous samples andmodel isochrones. From field stars of similar metallicity, the Hyades ([Fe/H] =+0.13) with no reddening has (m-M)_0 = 3.33 +/- 0.02 and M67, with E(B-V) =0.041, A_V = 3.1E(B-V), and [Fe/H] = 0.00, has (m-M)_0 = 9.71 +/- 0.02 (s.e.m),where the errors quoted refer to internal errors alone. At the extreme end ofthe age and metallicity scale, with E(B-V) = 0.125 +/- 0.025 and [Fe/H] = +0.39+/- 0.06, comparison of the fiducial relation for NGC 6791 to 19 field starswith (B-V) above 0.90 and [Fe/H] = +0.25 or higher, adjusted to the metallicityof NGC 6791, leads to (m-M)_0 = 13.07 +/- 0.09, internal and systematic errorsincluded.

The Unevolved Main Sequence of Nearby Field Stars and the Open Cluster Distance Scale