Whirligig
A whirligig is an overlapping set of four halfsquares: examples of a left whirligig and right whirligig appear below. There can optionally be an additional entry along the main diagonal. A whirligig is an inherently double form.
Left: Right:
D CCLAMPS
EL OHENRY KITSCH C
AAD MAGES SAMOA AY
ROUS BRAW BELL CDC
STREP ILL ELF MAIL
ISOPHANE ESCAPEE
RECTILINEARLY NONSEQUITUR
KNEEHOLE ALUMNUS
DAG RADAR TABU ATE
SELF BINE INS ROVE
POLIO NOV OD EDENS
PORTER SA N SYNTAX
FANTASM N