Guidelines

Some operations like select, crop, paint, move can use snapping to guidelines. There are dynamic guidelines which snaps to image bounds/center, and layer's bounds/center. There are also possibility to add custom guidelines. To add or delete custom guidelins first show Rulers - tap in main menu View->Rulers. Then 'drag' guideline from the ruler to add guideline. To remove it activate 'Move' tool and drag guideline out of canvas.