Intervals on a guitar take on distinctive shapes depending on whether or not the interval cross the boundary between the second and third string, here marked with a red line. This boundary is created due to the fact that there is a Major 3rd between the second and third strings, and a Perfect Fourth between all other strings.
Each yellow gold line represents an interval between two notes. This is exactly what an interval is the distance between one musical note to the next.
Perfect 1st (Unison)
Perfect 8th (octave)