For a given set of side lengths to create a triangle, the sum of any two sides must be greater than the length of the third side.
4 in, 7 in, 12 in = Not Possible
4 + 7 = 11 > 12 = False
4 + 12 = 16 > 7 = True
7 + 12 = 19 > 4 = True
6 mm, 6 mm, 10 mm = Possible
6 + 6 = 12 > 10 = True
6 + 10 = 16 > 6 = True
5 ft, 8 ft, 12 ft = Possible
5 + 8 = 13 > 12 = True
5 + 12 = 17 > 8 = True
8 + 12 = 20 > 5 = True
7 cm, 9 cm, 16 cm = Not Possible
7 + 9 = 16 > 16 = False
7 + 16 = 23 > 9 = True
9 + 16 = 25 > 7 = True
Hope this helps!