The Mixolydian Mode originally ascended from G to G'. This is a common mode for traditional music.
Scales in Score and Tab for common Mixolydian Modes as gifs or pdf
Use this table to construct the mode starting on any note, using the pattern of intervals below.
This pattern of intervals: T-T-S-T-T-S-T* is the characteristic of the Mixolydian mode, and is a Major mode similar to the Ionian but has a flattened 7th note.
Because the mode is characterised by its interval structure, you can start on any note and progress with the same intervals to produce a Mixolydian mode in that 'key'.
This would be 'D Mixolydian':
If a piece of music seems to be in the key of D, but all C#s are played as C naturals, it's probably 'D modal' or D Mixolydian.
Examples of tunes in Mixolydian Mode are Old Joe Clarke, The High Reel, Red Haired Boy and Banish Misfortune.
* T = Whole Tone, S = Semitone