Combining two letters in the current Sunday Stamps trip through the alphabet to feature four stamps issued by USPS this year to promote the STEM disciplines - Science, Technology, Engineering and Mathematics. I think the designs are intriguing, and make me think of some stamps issued in the 1970s.
These envelopes were sent as part of Jean's Pushing the Envelopes envelope exchange for April (I think).
For more links to S- and T-themed postage, head over to the links at Sunday Stamps.