The Extreme Significance of the Syllabograms JO and SI in Mycenaean Greek (Click to ENLARGE):

Ventris grids JO = YO & SI

This table is pretty much self-explanatory. About the only thing I wish to call to your attention is this: the syllabogram SI is not only highly flexible in its usages (5), but in its adaptability to expressing several alternative combinations of word endings: sij si ci. 

Much more on this later. I will soon explain how the Mycenaeans managed to adapt the Linear A morphed to Linear B syllabary to express the phonetic values of ancient Greek, which they actually did quite well, considering the serious restraints the Linear B script imposed on them.