321 Galactic fundamental-mode Cepheids with good B , V , and ( in most cases ) I photometry by ( ) and with homogenized color excesses E ( B - V ) based on ( ) are used to determine their period-color ( P-C ) relation in the range 0.4 < \log P \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox% { $ < $ } } } 1.6 . The agreement with colors from different model calculations is good to poor . – Distances of 25 Cepheids in open clusters and associations ( ) and of 28 Cepheids with Baade-Becker-Wesselink ( BBW ) distances ( ) are used in a first step to determine the absorption coefficients { \cal R } _ { B } = 4.17 , { \cal R } _ { V } = 3.17 , and { \cal R } _ { I } = 1.89 appropriate for Cepheids of intermediate color . The two sets of Galactic Cepheids with known distances define two independent P-L relations which agree very well in slope ; their zero points agree to within 0 \hbox { $ . ^ { m } $ } 12 \pm 0 \hbox { $ . ^ { m } $ } 04 . They are therefore combined into a single mean Galactic P-L _ { B,V,I } relation . The analysis of HIPPARCOS parallaxes by ( ) gives absolute magnitudes which are brighter by 0 \hbox { $ . ^ { m } $ } 21 \pm 0 \hbox { $ . ^ { m } $ } 11 in V and 0 \hbox { $ . ^ { m } $ } 18 \pm 0 \hbox { $ . ^ { m } $ } 12 in I at \log P = 0.85 . Agreement with P-L relations from different model calculations for the case [ Fe/H ] =0 is impressive to poor . Galactic Cepheids are redder in ( B - V ) ^ { 0 } than those in LMC and SMC as shown by the over 1000 Cloud Cepheids with good standard B,V,I photometry by ( ) ; the effect is less pronounced in ( V - I ) ^ { 0 } . Also the ( B - V ) ^ { 0 } , ( V - I ) ^ { 0 } two-color diagrams differ between Cepheids in the Galaxy and the Clouds , attributed both to the effects of metallicity differences on the spectral energy distributions of the Cepheids and to a shift in the effective temperature of the middle of the instability strip for LMC and SMC relative to the Galaxy by about \Delta \log T _ { e } \sim 0.02 at M _ { V } = -4 \hbox { $ . ^ { m } $ } 0 , hotter for both LMC and SMC . Differences in the period-color relations between the Galaxy and the Clouds show that there can not be a universal P-L relation from galaxy-to-galaxy in any given color . The inferred non-uniqueness of the slope of the P-L relations in the Galaxy , LMC , and SMC is born out by the observations . The Cloud Cepheids follow a shallower overall slope of the P-L relations in B , V , and I than the Galactic ones . LMC and SMC Cepheids are brighter in V than in the Galaxy by up to 0 \hbox { $ . ^ { m } $ } 5 at short periods ( \log P = 0.4 ) and fainter at long periods ( \log P \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $% } } } 1.4 ) . The latter effect is enhanced by a suggestive break of the P-L relation of LMC and SMC at \log P = 1.0 towards still shallower values as shown in a forthcoming paper .