Long a‐axes are a particularity of the crystal structures of the N3–‐poor lanthanide(III) nitride selenides with the composition M5NSe6 (monoclinic, C2/c, Z = 16; M = La: a = 5177.2(3), b = 1271.58(9), c = 785.49(5) pm, β = 89.386(7)°; M = Ce: a = 5128.4(3), b = 1260.24(8), c = 778.03(5) pm, β = 90.002(7)°; M = Pr: a = 5096.1(3), b = 1252.47(8), c = 772.61(5) pm, β = 90.705(7)°). Two [NM4]9+ tetrahedra sharing a common edge by forming angled [N2M6]12+ double units represent the main structural feature. These edge‐shared bitetrahedra reside in between layers of the defect Th3P4‐ like structure of the C‐type lanthanide sesquiselenides (M2Se3). An alternative description of the structure points out the half‐occupied cation positions (M10)3+. The discrete [N2M6]12+ units are linked with each others via four different selenide anions building up cationic$^2_{\infty}${[(N2M6)4Se14]20+} slabs. In the [100] direction they are stacked alternatingly with anionic $^2_{\infty}${[(M14Se34)(M10)2]20–} layers formed by Se2– polyhedra about the M3+ cations without any N3– ligands. Furthermore, these anionic layers offer channels filled up with (M10)3+ cations under half‐occupation conditions. The site occupation factor of 1/2 for (M10)3+ is plausible, because a full occupation of this site would reveal to high negative valence values for all Se2– anions (Se4, Se10, Se11 and Se13) that show contacts with (M10)3+.