# 74S	Later modification of 74 for even higher speed, at some cost in
# 	power consumption.  Effectively obsolete; use 74F.
# 
# 74LS	Combination of 74L and 74S, for speed comparable to 74 with lower
# 	power consumption.  Best all-round TTL now, widest variety of
# 	devices.
# 
# 74F	Fast as blazes, power not too bad.  The clear choice for high
# 	speed in TTL.  Availability and prices generally good.

I hate to disagree w/ a fellow as smart as Henry, but...

A few years back, I worked on a project using lots of high-speed
stuff.  (My part was slow & parallel, whew.) The mild-mannered designer
working on the fast (serial) stuff cussed & swore at 74F all the time.
It was the Harry Reams of TTL. One ultra-tiny power line glitch, one
hickup, one eyeblink across the board, and bang - the F had
toggled/counted/whatevered.  At times he swore it would count even
without any 5 volt supply ;-} You can guess what the "F" stood for....

They would do anything to push S or work around it to avoid using F. I
don't think I'd consider using F to replace S unless the consequences
were *fully* understood.......