The close packed structure is a crystal
structure in which the atoms (that are all of the same size) are as close together as possible. There are two crystal structure
s that are close packed: face centered cubic
(FCC) and hexagonal close packed
(HCP). They look remarkably similar but are not, in fact, the same.
The difference lies in the way the atoms are stacked. You can visualise this by imagining that you are stacking oranges, for example.
First lay down the bottom layer, so that the oranges are as close together as possible. Start with three oranges in an equilateral triangle and continue this arrangement to all sides.
The second layer is made by putting oranges on top of the equilateral triangles, covering the 'holes' between the bottom layer oranges, so that four-pointed pyramids are formed. Not all the 'holes' can be covered.
Coming to the third layer, you can choose: you can put the oranges vertically above those in the bottom layer, so that the stacking sequence is 'ABA'. This will get you the HCP structure. You can also put the oranges of the third layer over those holes under which there is no orange yet, resulting in an 'ABC' stacking sequence and the FCC structure.