By combining symbol layers, you can achieve a range of complex visual effects. For example, you can combine an instance of QgsSimpleMarkerSymbolLayerV2 with an instance of QgsVectorFieldSymbolLayer to display a point geometry using two symbols at once:

One of the main uses of symbol layers is to draw different LineString or PolyLine symbols to represent different types of roads. For example, you can draw a complex road symbol by combining multiple symbol layers, like this:

This effect is achieved by using three separate symbol layers:

Here is the Python code used to generate the preceding map symbol:

symbol =QgsLineSymbolV2.createSimple({})
symbol.deleteSymbolLayer(0) # Remove default symbol layer.

symbol_layer = QgsSimpleLineSymbolLayerV2()
symbol_layer.setWidth(4)
symbol_layer.setColor(QColor("light gray"))
symbol_layer.setPenCapStyle(Qt.FlatCap)
symbol.appendSymbolLayer(symbol_layer)

symbol_layer = QgsSimpleLineSymbolLayerV2()
symbol_layer.setColor(QColor("black...