Subject: When using the Sanger Method, how is the first base identified?

When you sequence DNA using the Sanger Method, each of the four test tubes
contains a complete set of nucleotides (plus a specific ddnucleotide).  Since
each test tube has all the normal nucleotides, the longest strand (which will
show up in the electrophoresis as the first base), should show up in every
column of the electrophoresis.  Thus, how do you determine which base is the
first one?

Re: When using the Sanger Method, how is the first base identified?