Layered transition metal dichalcogenides (TMDs) are commonly classified as quasi-two-dimensional materials, meaning that their electronic structure closely resembles that of an individual layer, which results in resistivity anisotropies reaching thousands. Here, we show that this rule does not hold for 1T-TaS₂—a compound with the richest phase diagram among TMDs. Although the onset of charge density wave order makes the in-plane conduction non-metallic, we reveal that the out-of-plane charge transport is metallic and the resistivity anisotropy is close to one. We support our findings with ab initio calculations predicting a pronounced quasi-one-dimensional character of the electronic structure. Consequently, we interpret the highly debated metal-insulator transition in 1T-TaS₂ as a quasi-one-dimensional instability, contrary to the long-standing Mott localisation picture. In a broader context, these findings are relevant for the newly born field of van der Waals heterostructures, where tuning interlayer interactions (e.g., by twist, strain, intercalation, etc.) leads to new emergent phenomena.

层状过渡金属二硫化碳（TMD）通常被分类为准二维材料，这意味着它们的电子结构非常类似于单个层的电子结构，这导致电阻率各向异性达到数千。在这里，我们表明该规则不适用于1T-TaS _2-一种TMD中相图最丰富的化合物。尽管电荷密度波序的开始使面内传导变为非金属，但我们发现面外电荷传输是金属的，并且电阻率各向异性接近于一。我们用从头算来预测电子结构的明显准一维特征来支持我们的发现。因此，我们解释了备受争议的1T-TaS _2中金属-绝缘体的转变与长期的Mott定位图相反，它是准一维不稳定性。在更广泛的背景下，这些发现与范德华异质结构的新生领域有关，在那里调整层间相互作用（例如，通过扭曲，应变，插层等）会导致新的出现现象。