Joints are simply cracks in rocks, a type of fracture structure where rock splits without obvious shifting of rock masses on either side of the crack. Widely distributed in shallow crustal rocks, joints are classified by origin into primary joints and secondary joints.

Primary joints form while the rock is being generated.

Secondary joints develop after rock formation, consisting of non-tectonic joints (mostly weathering joints) and tectonic joints.

All joints found in the Sanqingshan granite massif belong to tectonic joints, formed by crustal stress after the granite solidified. Granite commonly develops three sets of primary joints: vertical joints, flat-lying joints, and diagonal joints.