NKL (short for NanKai Lines) is a dataset for semantic line detection. Semantic lines are meaningful line structures that outline the conceptual structure of natural images. The NKL dataset contains 5,000 images of various scenes. Each of these images is annotated by multiple skilled human annotators. The dataset is split into training and validation subsets. There are 4,000 images in the training set and 1,000 in the validation set.