A flocculent spiral is one that has patchy structures in its disk rather than a well organised spiral structure. Spiral arms are very difficult to trace in these galaxies, often starting and stopping at random. This is in contrast to galaxies in which the spiral arms are very well defined, often called grand design spirals. It is thought that structure in the disks of flocculent spirals originate from regions of star formation that have been stretched into spiral patterns by the differential rotation of the galaxy. This is the key idea underpinning the self-propagating star formation model for the origin of spiral arms.