How does the Laundromat tell whose shirt is whose?

If you go to the laundry as infrequently as Gagler does, this can be a
vexing problem - he waits until he is on his next to last dress shirt,
then overwhelms the laundry guy with about 20 (or more items) at one
time. Considering a bunch of other guys, with similar habits, also drop
*their* laundry off on the same day this could be a problem; you know
the laundry guys just place everyone's things together in one big
washing machine!

Actually, to keep track of all of the shirts, etc., laundry workers
write on each shirt. Sometimes you can see it, sometimes you can't.
Another method is to write a series of letters or numbers in indelible
ink on the inside of a shirt collar; customers might keep the same
number while patronizing the same Laundromat.

Another method used by cleaners is called phantom marking. Three or
four letters or numbers are written on the inside of the collar using a
special ink that can only be seen using ultraviolet light. This number,
which corresponds to the number on your claim ticket is marked on the
collar. While some Laundromat's wash the shirts later in net bags, this
is done primarily to protect your garment rather than to separate it
from others.

After all of the laundry is dry, someone at the Laundromat passes each
shirt under an ultraviolet light and proceeds to sort it.