This is driving me crazy! Please, I just want to understand why this is happening! Okay, here's the situation.

I had lots of identically cut and measured rows. All that was left was to piece them together, alternating with rows of plain, cut strips. For lack of a picture:

###row made of squares###
###strip of plain fabric###
###row made of squares###
###strip of plain fabric###

Etc.

Why is it that the rows made of pieced squares gradually fall out of vertical alignment with the rows above (that have already been sewn together)? Again, they all started out the same length. The difference from one row to the next isn't hugely noticeable, but once everything is put together, the last, bottom row is probably an inch short when compared to the first, top row. Again for a visual:

##################### <-- top row
#####################
####################
####################
###################
################### <-- last row...now "shorter" than the top row, even though both started out the same length.

Does this question make sense? And if so, why does this happen? What can I do to prevent it from happening in the future?

I would appreciate any feedback on this issue.

Thanks for your time,
Bri