paper, we present a classification of the different types of variability observed in the X-ray emission of GRS 1915+105 and we demonstrate that all the variations can be accounted for in terms of switching between just three main spectral states. All the observations are found to follow this characterization, which is model-independent. The structure of the paper is the following. In Section 2, we present the extraction techniques and the tools used for the analysis. In Section 3, we proceed to the classification of the observations and present the evidence for the three basic states. In Section 4, we discuss these results in terms of physical models, in particular in connection with the disk-instability model presented in Belloni et al. (1997a,b). A forthcoming paper will include detailed spectral modeling for a precise (and model dependent) characterization of these states.

DATA EXTRACTION AND COLOR-COLOR DIAGRAMS
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We analyzed all observations of GRS 1915+105 made between January 1996 and December 1997 marked [PUBLIC]{} in the RXTE TOO archive (at heasarc.gsfc.nasa.gov), plus observation sequences 20187-02-01-00, 20187-02-01-01 and 20187-02-02-00, for a total of 163 observations. A list of the observations can be found, subdivided in classes (see below), in Table 1. Each observation consists 