Now we descend to the real situation, in which the period of the
waveform cannot be assumed to be arbitrarily long and
integer-valued. Suppose (for definiteness) we want to synthesize
tones at 440 Hertz (A above middle C), and that we are using a
sample rate of 44100 Hertz, so that the period is about 100.25
samples. Theoretically, given a very high sample rate, we would
expect the fiftieth partial to have magnitude 
compared to the fundamental and a frequency about 20 kHz.
If we sample this waveform at the (lower) sample rate of 44100,
then partials in excess of this frequency will be aliased, as
described in Section 3.1.
The relative strength of the folded-over partials will be on the
order of -32 decibels--quite audible. If the fundamental frequency
is raised further, more and louder partials reach the Nyquist
frequency (half the sample rate) and begin to fold over.
Foldover problems are much less pronounced for waveforms with only corners (instead of jumps) because of the faster dropoff of higher partial frequencies; for instance, a symmetric triangle wave at 440 Hertz would get twice the dropoff, or -64 decibels. In general, though, waveforms with discontinuities are a better starting point for subtractive synthesis (the most popular classical technique). In case you were hoping, subtractive filtering can't remove foldover once it is present in an audio signal.