text: when does 1+1 not equals to 2 ?   it depends how you define your number system. for example, the decimal number system in everyday use states 1 + 1 = 2 but if you define 1 differently, such as in base 2 maths (binary maths) then the previous answerer was correct in stating 1 + 1 = 10  alternatively if you define 1 as a symbol, not a number, then 1 could represent anything. e.g, you could define a whole new number system which goes 3,2,1,6,5,4... etc where 3 is the first number, 2 is the second number... etc this would be very confusing!! but the point i'm trying to make is that the concept of "one", "two" or "three" is not dependent on the symbols we use for them, 1, 2 ,3.