Just an addition to that already mentioned!
Flight Engineers and First Officers,
FE's only have 2 or 3 stripes, depending on their experience level, which means you won't see many 2 stripers' around these days!
Also, they have purple in between their stripes, signifying the fact that they are a Flight Engineer. (And not a First or Second Officer!)
The purple between the stripes originates back quite some distance.
I understand the Queen of England, following the sinking of the Titanic, awarded the Marine Engineers' aboard that ship the highest available award for bravery, for their efforts in keeping the ship afloat for as long as they did, and, for the majority, losing their lives in doing so!
That award, still being worn by marine engineers around the world in the form of the purple stripes in between their gold stripes, was passed on to the aviation world from the very beginning, and is still in normal use, in most 'knowledgeable' outfits.
Don't be confused by the FE wearing a wings brevet, in most companies it only signifies that he is a part of the technical flight crew! - although a large number of FE's do have a pilots licence to fly. (usually just a CPL or PPL for own use!
Our company allows the pilot licenced FE to fly the B747 for a t/off, circuit and landing in the simulator, just to keep up the "feel of the thing'.
Also, since we don't hire inexperienced FOs, we don't have any one or two striper FOs.
Just thought I'd mention it!
Cheers