###### Abstract
[20] demonstrated that thicker cement spaces can lead to high-stress concentration, specifically under the occlusal area of bi-layered zirconia crowns. Conversely, insufficient cement space can hinder the proper seating of the restoration, potentially resulting in a larger marginal gap, which can lead to biological complications [21]. Therefore, maintaining a cement thickness layer within the range of 50 \(\upmu\)m to 100 \(\upmu\)m is considered advantageous for ensuring the long-term performance of zirconia restorations. Veneering porcelain's are manually layered onto the zirconia substructure, although the press veneering porcelain technique is considered a viable alternative [22]. In addition to manual layering and pressing techniques, CAD/CAM veneering techniques for fabricating zirconia restorations have been developed to improve the chipping resistance of zirconia restorations [23]. [PERSON] et al. [24] evaluated the marginal gap of zirconia copings for single crowns before and after veneering with three different veneering techniques. The mean marginal gap, prior to veneer application, was 35 \(\upmu\)m; this value increased after the veneering process to 63, 50, and 51 \(\upmu\)m when using layering, pressing, and CAD-on veneering techniques, respectively. The production methods of zirconia prostheses significantly affect the marginal integrity of monolithic and bi-layered zirconia FDPs [2; 25]. Zirconia blocks are available as both pre-sintered and fully sintered blocks. From fully sintered zirconia, the desired restoration with the final dimensions can be milled with accurate fit results because fully sintered zirconia does not shrink upon sintering. Pre-sintered zirconia blocks can be milled to an enlarged size, where a shrinkage of approximately 25% occurs during the final sintering process of these blocks. This shrinkage must be compensated for at the milling stage [26; 27]. [PERSON] et al. [28] emphasized that zirconia restorations fabricated from fully sintered blocks using the hard machining method exhibited superior fit tolerance and marginal adaptation compared with zirconia produced by soft machining. This improper fit could be attributed to the shrinkage that occurs in the pre-sintered zirconia after sintering. In a recent study that evaluated the marginal discrepancy of monolithic zirconia produced using different CAD/CAM systems, the marginal fit of all groups was within the clinically acceptable range [29]. It was concluded that monolithic zirconia milled using 5-axis and dry milling exhibited excellent marginal adaptation. As monolithic zirconia has been used to overcome the technical complications of veneer ceramics in layered zirconia restorations, the marginal and internal discrepancy data of monolithic 3-unit zirconia FDPs compared with bi-layered FDPs are limited. Therefore, this study aimed to evaluate the influence of the restoration design (monolithic vs. bi-layered) and manufacturing technique on the marginal discrepancy and internal fit of layered and monolithic zirconia FDPs. The null hypothesis that was tested was that the restoration design and manufacturing technique had no significant effect on bi-layered and monolithic zirconia restorations.