by themselves

by (one)self

1. Alone. Because there's usually so much going on with the kids, I really enjoy spending a quiet Saturday night by myself.
2. Through one's own efforts alone. You can't move that table all by yourself! Here, let me help you. My little girl loves to get dressed by herself, so tutus and cowboy boots have become the norm around here.

by themselves

 
1. with help from no one else. Do you think they can do it by themselves? Mike and Max cannot lift the piano by themselves.
2. with no one else present; alone (of two or more people). They are sitting there by themselves. Let's sit with them. They enjoy spending the evening at home by themselves.
See also: themselves
References in classic literature ?
It seems the Spaniards had kept possession of the main apartment, and had made a place for the three Englishmen, who, since their last mutiny, were always quartered by themselves, and could not come at the rest.
Within these virtual circles, teens become part of what they regard as a cool Internet practice and, at the same time, intensify feelings of being connected to friends, even when sitting by themselves doing homework, Kraut says.
It is simply that, by themselves, they cannot account for the aesthetic success of these paintings.
The Matadors (15-8) are by themselves in first place in the Big Sky Conference by 1 1/2 games at 8-2.
There's something deeply subversive, however, in the notion that individuals might actually be trusted to cope by themselves with the hazards to be found in their file cabinets and laser printers.
owner, Waller discovered franchising allows owners to be in business for themselves, but not by themselves.
Don't pull them out and put them in schools by themselves.
com is addressing with its online networking service, as registered members not only have an online forum where they can communicate, they can also post links to industry related websites, news headlines and articles written by themselves on the website.
The chief also believes permits should not be issued when there are alternatives to reducing the danger, such as jewelers having merchandise transported by professional couriers rather than by themselves, or doing the work during the day rather than late at night.
Participants then viewed videotapes of these sessions by themselves in separate cubicles.