Variations: From the 1920s, recipes add more orange juice, such as the juice of a quarter orange (McElhone, 1927); that fruitier drink is the Bronx most bars make now.
Boothby gives a third each of gin and the two vermouths and “about a barspoon” of orange juice; the amounts are scaled to a 3 oz drink. He doesn’t say how to mix it; Jack’s Manual (1910) shakes it.
The ratio
1 : 1 : 1
In parts, a Bronx is 1 gin, 1 vermouth and 1 vermouth. Plus a touch of orange. Change one part and you have a cousin; the closest are below.