Slices of triangular alphabets
Python 2, 101 99 bytes

r=range(input())
for x in sorted(r,key=lambda x:x*-(x&amp;2)):print bytearray([97+i,32][i&lt;x]for i in r)


xsot saved two bytes by realizing x*-(x&amp;2) suffices as a sorting key — the bottom half of the resulting image is unaffected due to sorted guaranteeing a stable sort.