Autotiling
Automatically picks the correct tile graphic for each cell based on which neighbours it has, so edges and corners fit together without hand placement.
The technique decides which variant of a tile to draw by reading the surrounding cells — usually as a bitmask over four or eight neighbours. A 4-bit mask yields 16 cases; an 8-bit mask gives 47 or 256 depending on the rule set. The level designer no longer places every corner and edge by hand; you paint the fill and the engine supplies the matching border art. Tilesets in Tiled, Godot and RPG Maker ship with built-in support.
Related approaches include Wang tiles, which encode matching rules in a tile's edges, and the dual-grid trick that offsets the grid by half a cell to avoid the 47-tile set entirely.