Similar to our solar system, extra-solar systems likely also contain a diversity of planets, and planets around the same star likely have (or had) interactions with each other. Therefore, the relations of planets inside the same system conveys important clues on their origin and evolution path. In this talk, I will present several such relations and discuss the theoretical implications. The first part of my talk focuses on the intrinsic architecture of the inner (<1 au) planetary system revealed by Kepler. In the second part, I will extend the “planet ladder” to larger separation, to correlate the inner small planets with the outer giants. In the end, I will switch gear to gravitational microlensing, presenting the first pair of microlensing planets near mean-motion resonance and discussing its role in understanding the architecture of outer planetary system down to lower masses.