Slack Pattern

It's been awhile since I've sewed any clothes for myself....so, I went to the local fabric store to purchase some patterns.  Is it me or what....the pattern sizes don't match the size of ready made clothes.  What's up with that?  So, I took an old pair of slacks that still fit, and took them apart to use as a pattern. After cutting the new slacks out, I put the pattern (old slacks) in a large baggie and marked the bag 'slacks.'