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"Siniestra" is a form of "siniestra", a noun which is often translated as "left hand". "Diestra" is a form of "diestra", a noun which is often translated as "right hand". Learn more about the difference between "siniestra" and "diestra" below.
la siniestra(
see
-
nyehs
-
trah
)A feminine noun is almost always used with feminine articles and adjectives (e.g., la mujer bonita, la luna llena).
1. (literary) (anatomy)
A word or phrase restricted in usage to literature or established writing (e.g., sex, once upon a time).
a. left hand 
Antiguamente existían prejuicios hacia la gente que escribía con la siniestra.In the past, there was prejudice against people who wrote with their left hand.
2. (literary) (side)
A word or phrase restricted in usage to literature or established writing (e.g., sex, once upon a time).
a. left 
A la siniestra del castillo había un bosque frondoso.To the left of the castle lay a thick wood.
la diestra(
dyehs
-
trah
)A feminine noun is almost always used with feminine articles and adjectives (e.g., la mujer bonita, la luna llena).
1. (anatomy)
a. right hand 
Aunque Pablo es zurdo, puede escribir muy bien con la diestra.Even though Pablo is left handed, he can write pretty well with his right hand.
2. (right side)
b. right-hand side 
Los contrabajos se encuentran a la diestra de la orquesta.The double basses are on the right-hand side of the orchestra.