Wikipedia
The long-short romanization system is a romanization system for northern Wu dialects, particularly the Shanghai dialect and the Suzhou dialect. The system is called "Long-short" because the system distinguishes between long vowels, which are written with two vowels, and short vowels, which are written with one. In some dialects, the short vowels are pronounced with a following glottal stop, making the distinction between short and long vowels more important.
A distinctive characteristic of Wu dialects is their retention of the Middle Chinese "muddy voice" initials, so that Wu is one of the few Chinese dialect groups with a three-way distinction between voiced, voiceless and aspirated consonants. Because other Chinese varieties such as Mandarin and Cantonese distinguish only between unaspirated and aspirated initials, romanization systems used for those dialects use the convention of writing, for example, the unaspirated as "b" and the aspirated as "p". However this format is insufficient for the thrree-way distinction preserved by Wu, so the long-short romanization uses an "h" to show aspiration (so that is written "ph").