The coenagrion spur

Several fairly similar damselflies - common blue, azure and variable blue - all visit the garden from time to time.I usually rely on the black markings on the second segment (S2) of the abdomen plus the shape of the ante-humeral stripes on the thorax to identify them. I've now learnt about the coenagrion spur -  the small black line below the ante-humeral stripes on the side of the thorax. This can be helpful if you can't see S2 as the common blue doesn't have the spur. The spur is quite clear on this image of the male azure damselfly(coenagrion puella).