Answer:
When something is in layers stacked horizontally, the ones at the top are the youngest and the ones at the bottom are oldest.
Step-by-step explanation:
Usually applied to rock layers for relative dating purposes the layers at the bottom are deposited first and the ones at the top last.
ex.
---------
Layer d
-------------
Layer C
----------------
Layer B
-------------------
Layer A
-----------------------
Layer A was deposited first, then B, Then C and then D. This can be applied to much more then rocks though. In most houses the top floor was built last and the ground flour was built first, you can tell because using the law of superposition you can see that the second floor is above the ground floor, therefore the ground floor was built first and the second floor was built after.