In the above drawings, DE, DF, and EF are the midsegments of triangle ABC.
The sides of triangle ABC are the bases. 

The four triangles ADE,DBF,EFC and FED are all congruent and
they are all similar to ABC. Each midsegment is one-half of the
side (base) to which it is parallel.  

Therefore if you know the length of the midsegment, then the 
length of the base that it is parallel to is twice the
length of that midsegment.

Edwin