につき has two main uses. First, it expresses a reason or cause, similar to ため or ので, but it carries a formal, business-like tone. You often see it in notices, announcements, and official documents, such as '本日は都合につき休業します' (Due to circumstances, we are closed today). In speech, it sounds stiff, so use ので or から instead.

Second, it indicates a rate or allocation, meaning 'per' or 'for each'. For example, '大人一人につき千円' means '1,000 yen per adult'. This usage is common in pricing, rules, and distributions. It attaches directly to a noun, like 一人, 日, or 枚.

In both uses, につき is not flexible in form: it follows a noun and is fixed. It is not used with verbs directly; for example, you cannot say '行くにつき' to mean 'because I go' — that would be 行くので. For reasons with verbs, use ため.