Look between opposite vertices. That is where you will find the diagonal.

The measure of the diagonal depends on which diagonal you are interested in and the details of the rhombus in question. The only thing you know for sure is that each diagonal is a perpendicular bisector of the other. In general, the two diagonals are of different length. They will only be the same length if the rhombus is a square.

The measure of the diagonal depends on which diagonal you are interested in and the details of the rhombus in question. The only thing you know for sure is that each diagonal is a perpendicular bisector of the other. In general, the two diagonals are of different length. They will only be the same length if the rhombus is a square.