49 CFR 356.7 - Tacking.
Unless expressly prohibited, a motor common carrier of property holding separate authorities which have common service points may join, or tack, those authorities at the common point, or gateway, for the purpose of performing through service as follows:
(a) Regular route authorities may be tacked with one another;
(b) Regular route authority may be tacked with irregular route authority;
(c) Irregular route authorities may be tacked with one another if the authorities were granted pursuant to application filed on or before November 23, 1973, and the distance between the points at which service is provided, when measured through the gateway point, is 300 miles or less; and
(d) Irregular route authorities may be tacked with one another if the authorities involved contain a specific provision granting the right to tack.