The boundary between two crystal grains can decompose into arrays of facets with distinct crystallographic character. Faceting occurs to minimize the system's free energy, i.e., when the total interfacial energy of all facets is below that of the topologically shortest interface plane. In a model Al-Zn-Mg-Cu alloy, we show that faceting occurs at investigated grain boundaries and that the local chemistry is strongly correlated with the facet character. The self-consistent coevolution of facet structure and chemistry leads to the formation of periodic segregation patterns of 5-10 nm, or to preferential precipitation. This study shows that segregation-faceting interplay is not limited to bicrystals but exists in bulk engineering Al alloys and hence affects their performance. Grain boundaries (GBs) are regions connecting adjacent crystals with different crystallographic orientations. GBs are a type of lattice imperfection [1,2], with their own structure and composition, and as such impact a material's mechanical and functional properties [3]. Structural motifs and phases formed at chemically decorated GBs can be of a transient nature [4-7] or are local thermodynamic structural-chemical equilibrium states [8-11]. General GBs exhibit a wide range of local crystallographic structure and facet motifs. In-plane GB structural motifs arise naturally from topological and bonding constraints associated with the adjacent crystals. An initially flat GB can decompose into a series of sequentially arranged facets, driven by the reduction in total interfacial energy. This counterintuitive transition, where the total length of the faceted interface exceeds that of the shortest interface, originates from the interplay between the interfacial energy of individual facets and their crystallographic character. Altogether, the longer interface can be of overall lower free energy [12-15]. GBs often see the segregation of solutes or impurities, driven by a reduction of the interfacial energy according to the Gibbs adsorption isotherm, and are favorable sites for heterogeneous nucleation of precipitates. These can form during quenching [16] or during natural or artificial aging in Al alloys [17,18]. Diffusion, segregation, and precipitation are intimately related to the local GB structure, which can result in strain localization, intergranular fracture, and corrosion [19]. The interaction between GB composition and structure was mostly studied on well-defined bicrystals at microscopic [5,14,20-22] and atomic scales [11,12,23,24]. However, such model samples can be far from representative when compared to bulk materials processed via conventional routes. Here, we reveal the interplay between composition and structure at GBs in an engineering material by using aberration-corrected scanning transmission electron microscopy (STEM), atom probe tomography (APT) and atomistic simulations. Near Σ5, Σ11, and Σ13a coincident-site lattice boundaries were studied in a coarse-grained Al-6.22%Zn-2.46%Mg-2.13%Cu alloy (wt. %). These boundaries were selected because specimens for high-resolution STEM require that both grains are along a common low-index zone axis to image individual atomic columns. The cast material was homogenized, hot rolled, and then solution heat treated at 475°C, followed by water quenching. Details are in the Supplemental Material [25]. The investigated GBs decompose into facets ranging in length from micrometers down to only a few nanometers. A distinct segregation and precipitation behavior is observed on the different facets. Our findings are of interest primarily for two reasons: first, little is known about the interplay of GB chemistry and faceting in general and in particular in engineering Al alloys; second, segregation and precipitation on specific GB facets influence strain localization, damage initiation and corrosion, and thereby materials' macroscopic properties. The first investigated GB was a near Σ11 GB from the as-quenched Al-Zn-Mg-Cu alloy. APT and STEM results are presented in Fig. 1. The GB, aligned edge-on, is