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It is believed that the strong magnetostriction of iron-gallium alloys around 19 at.% and 27 at.% Ga relies on formation of precipitates in a metastable body-centered cubic (bcc)-base matrix showing classical D03 ordering. In order to develop a more comprehensive understanding of the interrelationship between the different phases in the Fe-Ga system, the present work examines the crystallographic characteristics of three Ga-rich monoclinic intermetallic phases: metastable Fe13Ga9, and equilibrium α-Fe6Ga5 and Fe3Ga4. It is shown that the crystal structures of these phases all can be understood as a coherent set of ordered vacancy-containing bcc-derivative superstructure phases. A formal procedure to derive these structures from the bcc structure is presented and adapted to quantify the similarity with the bcc structure in terms of strain (of the unit cell) and internal displacements of the atoms. This procedure is applied to structure data from first-principles calculations and X-ray diffraction. The analysis demonstrates that within the group of vacancy-containing bcc-derivative crystal structures, certain subgroups of derivatives of the ω-phase structure or the B8 structures can be proposed. These subgroups are relevant for the structures of Fe13Ga9 and α-Fe6Ga5, which can be related, in two nearly equally close ways, with the bcc structure. Physical relevance of the formal derivation of the monoclinic crystal structures is demonstrated by observation of corresponding orientation relationships of grains/domains of α-Fe6Ga5 and Fe3Ga4 in a two-phase alloy. The insights in the crystal structures of Fe13Ga9, α-Fe6Ga5 and Fe3Ga4 are further discussed regarding implications for the lower-Ga content “classical” bcc phases.