Complex sentence: Like double "s", it is pronounced as an (see IPA), but in standard spelling, it is only used after long vowels and diphthongs, while "ss" is used after short vowels.
Split into simpler sentences: Like double "s", it is pronounced as an (see IPA). However, in standard spelling it is only used after long vowels and diphthongs, while "ss" is used after short vowels.