This is a guitar tuner for a drop D tuning, which is used in a lot of genres ranging from classical, to country to metal. It is a standard E tuning with the low E string dropped to a D, therefor its name drop D.
Guitar Tuner Standard E
Here is a guitar tuner for the E Standard tuning, which is the most commonly used tuning. The low E string (6th string) is the closest to the ceiling, so the biggest one. The high E string (1st string) is the closest to the floor, so the smallest one.
Bass Tuner Standard E
Here is a bass tuner in standard E tuning: E, A, D, G. That is the usual tuning for any type of bass guitar. The string #4 is the biggest one, but also the closest to the ceiling. The string #1 is that smallest one and the closest to the floor.