Quote from the Official Standard

"Set on low, jutting out rather straight and then turning downwards.  Round, smooth and devoid of fringe or coarse hair.  Moderate in length - rather short than long - thick at root, tapering quickly to a fine point.   Downward carriage (not having a decided upward curve at end) and never carried above back."

Correct tail 

"Set on low, jutting out rather straight and then turning downwards."

"Moderate in length - rather short than long - thick at root, tapering quickly to a fine point."

Screw tail - not a fault

Gay tail (fault)

".... and never carried above back."