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Fifteen words put R first and U third, and eighty percent of them open RO.
O takes the second box in twelve of the fifteen. A, E and H manage one each, so in practice the second letter is O unless something has already ruled it out.
REUSE is the useful exception and the only entry with E in the second box. RHUMB puts an H there, RAUPO an A, and neither is a word you would produce. So a confirmed second letter that is not O leaves you with REUSE and two long shots.
ROUND, ROUTE, ROUGH and ROUSE are all common and all sit in the same branch, so the fourth and fifth boxes carry the whole decision. P, S and T tie for the fourth box at three words each, E closes four of the fifteen, and nothing on the page repeats a letter.
Both are common and they differ in the last two boxes. ROUND tests N and D, neither of which appears much elsewhere here.
On this list it is a single five letter entry. It is the only word here without O in the second box that anyone would guess.
A line crossing all meridians at the same angle, used in navigation. It is the only entry here with H in the second box.