Tank sizes

[Davep]

When looking at the forums is see a lot of people referring to the tank sizes in gallons high, I live in Ireland and never heard tank sizes referred to in this way, my tank is a 55 gallon measuring 120-50-40 cm
Can someone please fill me in on the meaning

 

[DragonFish71]

Some tanks can come in long or high.

Take a 20 gallon, there is the 20 long: 30 1/4 x12 1/2 x12 3/4
and there is a 20 high:24 1/4 x12 1/2 x16 3/4

Chart of Tank Sizes

 

[tnfinfan]

Not to mention squares, cubes, and hexagons...