Four bodies, four right angles: two oppositions crossing each other squarely. The most demanding of the configurations here to satisfy, and the rarest in the catalogue.
A grand cross needs two independent oppositions (four bodies, none of them the same body twice) where every body also squares both bodies of the other pair — four 90° angles and two 180° angles at once, all inside orb simultaneously. It contains two T-squares, one for each apex; this site reports the four-body cross and folds the two three-body T-squares it's built from into it rather than listing all three.
Like the T-square, a grand cross drawn from planets near sign cusps can span more than the four signs of a single modality, but the typical case is exactly those four — the two axes of the cardinal, fixed or mutable cross, occupied at both ends.
This is the configuration most likely to be a near-miss rather than a hit: four bodies each within 6–7° of a right angle to two others is a demanding coincidence, and a chart that just misses it by a degree on one corner will show a T-square instead, with nothing marking how close the fourth leg came.
Traditionally attributed
The tradition's reading is unusually consistent across sources: sustained, structural tension with no single point of release — traditionally the hardest configuration to have and, by the same token, the hardest to talk yourself out of ever grappling with.
Listed because it is what the tradition says, not because this site can check it. Everything above this line is geometry; everything in it is convention.