What does .79 of an inch look like on a us ruler?

You will not be able to see 0.79 of an inch on a US ruler. The best you can hope for is 0.8 inches. On a ruler, an inch is usually divided into 10 or 16 parts - the smaller, unnumbered marks between the inch marks. You can get 32 divisions on tape measures but they are unusual on rulers.

Select a part of the ruler in which each inch is divided into 10 parts. Eight of them will make 0.8 inch. You will not be bale to tell the difference between 0.8 inch and 0.79 inches by eye!