The parties are bound to perform at the same time so that the performance of each one is dependent on the simultaneous performance of the other.
Example: B hands cash to C with one hand and takes goods with the other.
An event, other than a lapse of time, must occur before the other party has an absolute duty of performance. Example: A must deliver the goods before B has a duty to pay.
An event discharges an already existing absolute duty of performance (very rare).
1. No Substantive Difference
2. Procedural Difference
a. The party to whom the duty is owed must prove that a condition precedent has occurred.
b. The party who owes the duty must prove that the condition subsequent has occurred.
The Restatement (Second) does not distinguish between conditions subsequent and precedent.