Short answer. Collateral evidence is generally not allowed, but there is a real exception. Rule 128, Section 4 requires evidence to relate to the fact in issue enough to induce belief in its existence or non-existence, and bars evidence on collateral matters — except when it tends, to any reasonable degree, to establish the probability or improbability of the fact in issue.
What the law says
Evidence must have such a relation to the fact in issue as to induce belief in its existence or non-existence.
Rule 128, Section 4 — Relevancy; collateral matters. Read the full provision →
What the law says
Evidence on collateral matters shall not be allowed, except when it tends in any reasonable degree to establish the probability or improbability of the fact in issue.
Rule 128, Section 4 — Relevancy; collateral matters. Read the full provision →
What relevancy actually requires
Section 4 spells out what Section 3's relevance requirement means in practice: evidence must have such a relation to the fact in issue as to induce belief in its existence or non-existence. It is not enough that evidence is tangentially connected — it has to move the fact-finder's belief about the actual fact in dispute. This is a functional test, not a formal one: the court asks whether the evidence, taken with everything else in the case, actually makes a party's version of the disputed fact more or less believable, rather than simply asking whether the evidence relates to the case in some general or abstract sense.
The general bar on collateral matters
Building on that, the section states plainly that evidence on collateral matters shall not be allowed. Collateral matters are things outside the direct fact in issue — allowing unlimited evidence on tangents would burden trials with information that does not actually help resolve the disputed fact. This default exclusion keeps trials focused on the actual dispute rather than expanding into every tangentially connected topic a party might wish to raise. The bar binds the offering party, not the judge's own curiosity; a court may not admit collateral matters simply because they strike it as interesting or colorful if the matter fails the reasonable-probability test the next section describes.
The reasonable-probability exception
The bar is not absolute. Section 4 permits collateral evidence when it tends, in any reasonable degree, to establish the probability or improbability of the fact in issue. This means a party can sometimes use seemingly collateral evidence, so long as it can be shown to reasonably bear on the central fact the case turns on. In practice, this exception often covers things like a person's habit, a place's condition at a relevant earlier or later time, or a party's opportunity to do something, none of which are the fact in issue itself but each of which can reasonably shift the likelihood that the fact in issue is true.