2on2 (read two on two) means two hands on two hands or feet, or two feet on two hands or feet; when holding it hand to hand 2on2 can either mean having the hands separated or on top of each other (“knot”).
2on1 (read two on one) means two hands on top of each other (“knot”) on one hand or foot.
1on2 (read one on two) means one hand on two hands or feet on top of each other (“knot”) or one foot on two hands or feet on top of each other.
1on1 (read one on one) means one hand on one hand or foot, or one foot on one hand, or one foot on one foot. For example, the easiest and shortest way to describe a “One Arm Handstand held with one extended Arm of the Base” would be: High 1on1 HS, which does not define the base position though, so it could as well refer to a seated or a lying position, thus more precisely Standing High 1on1 HS. But since the standing position is the default, the standard position in Acro, and since the most essential 1on1 position is the One on One Handstand (less often a footstand or lever), “1on1” alone lets acrobats most often think of a standing high 1on1 handstand.
Mono, Extended Mono is a different and less condensed way of describing One Arms. An “Extended Mono” is meant to be the One on One Handstand on straight arm, that is commonly used in non-gymnastic acro circles. We do not suggest using it, since it says less with more. Extended here refers to an extended arm of the base, which we use to define as “high” or “straight” arm, while “Mono” could either refer to one arm/foot of the base or one arm/foot of the top; it does not make it really clear that it is One on One, that both partners are balancing with one arm/foot only. Also while usually being used to describe a handstand, it doesn’t make this clear either, as it could as well be about a One on One Footstand or One on One Straddle Support. While obviously it will be clear what is meant by context (just as 1on1 as well refers to a Handstand most of the time), when it has to be described precisely, it is still shorter and more precise to say “High 1on1 (HS)” (if it is about a Handstand specifically) than to say “Extended Mono (HS)”, by which it is still unclear by the words if it is the base holding it with one arm or the top or both. A 2on1 Handstand is also “mono” on behalf of the top and also extended, and a 1on2 Handstand is “mono” on behalf of the base. But only “1on1” clearly signifies that both partners balance on one arm or leg.
Short
Extended is a different way to refer to straight or high arms used in non-gymnastic acro circles, which we do not recommend to use. Referring to the arms of the base as being “extended” is confusing, because when speaking of the position of the arms, we tend to think of shoulder extension, not elbow extension; extended arms refer to a very different position with the arms in extension (behind the back), while what this is really about is straight arms and most often, when speaking of a standing position, in shoulder flexion, not in extension. Thus, when acro gymnasts speak of e.g. Extended 1on1, they may refer to a position in which the base is bend over forward and holds the top on one arm held vertically in shoulder extension behind his back in a forward-fold, because here the shoulder holds the arm in an actually extended position, while a regular Straight/High Arm 1on1 has the shoulder in a flexed position.
Bent Arms, a position of the arms of the base partner, which, in a lying position, equals the starting position in a floor press, with the elbows close to the torso, while in a seated, kneeling, lunging or standing position equals the position of the arms e.g. in a shoulder press, with the hands held about shoulder width apart, also simply called “low” as opposed to straight arms as “high”.
Straight Arms refer to the arms of the base partner, holding them straight, not bent, either in a 90° angle from the body/floor when lying, or in a 180° angle and in an overhead position (shoulder flexion) when in a seated, kneeling, lunging or standing position. Also simply called “high” as opposed to bent arms as “low”.