Double digit addition refers to adding two 2-digit numbers mentally starting from 10 to 99. The result of adding two-digit numbers may either be a two or three-digit number. By adding double digit addition sums, children learn to use blocks of tens and ones for addition problems. When adding two-digit numbers, children tend to look at the ones and decide if they can regroup them for a ten. This helps kids to think of numbers in terms of both ones and tens instead of just ones. Young learners can refer to and download simple double digit addition worksheets PDF given below.