Clew is a chart plotting program. Connect a GPS to your laptop, load some maps, and watch the dots crawl across the map as you drive, sail, etc. It currently works with maps scanned/photographed into the computer and any GPS with NMEA output.
1) The lower corners of a square sail, "and the after lower corner of a fore-and-aft sail." (Underhill) 2) The lines attached to those corners and used to haul the clews upward to the yard for furling.