Doubles are adding the same number together, as with 5+5=10.
With doubles +1, the numbers must be in beside each other in counting order going up or down (ex: 4, 5, 6 and 9, 8, 7). Do the SMALLEST number, double it, then add one. So, 9+8 would be 8+8=16, +1=17. You must know your doubles facts before this strategy is useful.
