Multiples: a pair of identical twins who were once multiples

When we see  a pair of identical twins, what are we seeing? Are they truly the result of a single zygote splitting into two?  Or did that zygote split again once, creating triplets, or twice, creating quads?  Its a varied and confusing picture  when you start to think of additional splits. This image shows how triplets can be formed - 3 single zygotes, one single zygote and an MZ pair and a single zygote that splits twice.

When one zygote splits twice but two of the tiny embryos die, the resultant twin pair will share the same genes, as monozygotic twins all do, but they will both be womb twin survivors.

And then there is the question of the timing of the various splits - it's all crucial to the outcome......

Multiples: How to tell twins from triplets

In Mary-Kate and Ashley Olsen, we have a good example of fraternal twins who are different from each other in significant ways - which,  if we look carefully, may betray their pre-birth origins as a set of multiple embryos. They were so similar as very young children that they were able to play the same role interchangeably in Full House.

The directors and producers of Full House did not want people to know that Michelle was played by a set of twins, so the girls were credited as Mary-Kate Ashley Olsen for most of the show's run. It wasn't until the final season that they were credited Mary-Kate and Ashley Olsen. Despite the fact that the Olsens are fraternal twins, their physical appearances were nonetheless similar enough so that few could tell the difference between the two.....

This from Squidoo:

Many of these things listed below were used to tell them apart when they were younger. Some can still be used now to tell them apart.

• Mary-Kate is left-handed. Ashley is right-handed.
•  Ashley is a little bit taller than Mary-Kate.
•  Ashely has a freckle above her lip