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 WHIRLIGIG
D CCLAMPS
EL OHENRY
AAD MAGES
ROUS BRAW
STREP ILL
ISOPHANE
RECTILINEARLY
KNEEHOLE
DAG RADAR
SELF BINE
POLIO NOV
PORTER SA
FANTASM N
RIGHT WHIRLIGIG
KITSCH C
SAMOA AY
BELL CDC
ELF MAIL
ESCAPEE
NONSEQUITUR
ALUMNUS
TABU ATE
INS ROVE
OD EDENS
N SYNTAX