Dipthongs. The signs are written in the sounded order. For conveneince the long 'i' is expressed by the large circle in some words. This omits the necessity of ticking it below the outline as some may need to be in your writing (although I've never used them, just never bothered, but probably would help).
"Gregg Shorthand had achieved the most extraordinary success ever attained by any system in the history of shorthand." John Robert Gregg, 1922