I use mixed units systems... for example, some values are easier to visualize in one system vs the other.
I was in 5th grade when Oz officially went metric in 1972, so I learnt both systems...
Code:
12 inches = 1 foot
3 feet = 1 yard
5.5 yards = 1 rod = 1 pole = 1 perch
4 rods = 1 chain
10 chains = 1 furlong
8 furlongs = 1 mile
3 miles = 1 league
6 feet = 1 fathom
25 links = 1 rod
40 sq rods = 1 rood
4 roods = 1 acre
640 acres = 1 sq mile
...bloody useful stuff to clutter up one's brain (...or what's left of it...)...