What is a semi truck? Tractor unit, semi-trailer and trailer defined

A semi truck is a tractor unit coupled to a semi-trailer, a trailer that rests partly on the tractor. United States federal rules define the two halves in 49 CFR 390.5T, as published in the eCFR on 5 October 2026: a truck tractor is “a self-propelled commercial motor vehicle designed and/or used primarily for drawing other vehicles”, and a semitrailer is “constructed so that some part of its weight rests upon the self-propelled towing motor vehicle”. In European Union law, Council Directive 96/53/EC (Article 2) calls the combination “an articulated vehicle consisting of a motor vehicle coupled to a semi-trailer”, and the towing vehicle has its own bodywork code, BC, “Tractor unit for semi-trailer”, in Annex I to Regulation (EU) 2018/858. Semi truck, 18-wheeler, tractor-trailer and big rig are everyday names; none of the definitions compared on this page uses them.

Status as of 8 October 2026. Every text was read on that day, in the version named in the second table and the source list; the EU texts in English, German and Spanish, the UN R.E.3 in English and Russian. The eCFR marks 49 CFR 390.5 as suspended indefinitely; the parallel section 390.5T, quoted here, has the same wording for every term used on this page. Turkish and Spanish legal texts are given in our translation; quotations keep the original words, followed by the translation in brackets.

Everyday words and the terms in the texts

Each market has its own everyday words for the vehicle. The table sets them beside the terms used in the texts read for each market or language: the national texts, the EU texts in their English, German and Spanish versions, and for Russian the Russian version of R.E.3.

Neither tır nor dorse occurs in the current text of Türkiye’s Law No. 2918 or of the Road Transport Regulation (our search of both texts on 8 October 2026); both use çekici and yarı römork. The Regulation’s definition of a birim taşıt (unit vehicle) begins with “Yarı römorkuyla birlikte çekiciyi” (a çekici together with its yarı römork) (Article 4(1)(ğ)).

No text compared here defines an 18-wheeler or fixes a number of wheels for a semi truck. On a five-axle tractor and semi-trailer with single wheels on the steering axle and dual wheels on the other four axles, the wheels add up to 2 + 4 × 4 = 18 (our count); other axle or wheel layouts give other totals. Axle layouts are explained in axle configurations explained, and day cabs and sleeper cabs in truck cabs.

How the texts define the tractor unit, the semi-trailer and the combination

The same vehicle is defined in each of these texts for a different purpose: type approval in Regulation (EU) 2018/858, weights and dimensions in Directive 96/53/EC, CO2 standards in Regulation (EU) 2019/1242, the construction of vehicles in R.E.3, and road traffic, vehicle and transport rules in the national texts.

What the towing vehicle may carry. The EU and US texts describe a vehicle designed “exclusively or principally” (Regulation (EU) 2018/858, code BC) or designed and/or used “primarily” (49 CFR 390.5T) for towing. The Turkish texts define the çekici by the load it does not carry: “yük taşımayan” (carrying no load) in Law No. 2918, “bunlar olmadan eşya taşıyamayan” (unable to carry goods without them) in the Regulation. 23 CFR 658.5 calls the truck tractor “The noncargo carrying power unit that operates in combination with a semitrailer or trailer” and makes two exceptions: a truck tractor and semitrailer transporting automobiles may carry motor vehicles on part of the power unit, and a truck tractor with a dromedary unit, pulling a semitrailer that carries Class 1 explosives and/or munitions-related security material, may use the dromedary unit to carry a portion of the cargo.

How much of the semi-trailer rests on the tractor. The texts use general words: “a substantial part of its weight and of the weight of its load” (Directive 96/53/EC), “a substantial vertical load” (Regulation (EU) 2018/858, code DA), “una parte sustancial de su masa” (a substantial part of its mass, Reglamento General de Vehículos), “taşıdığı yükün ve kendi ağırlığının bir kısmı” (part of the load it carries and of its own weight, Law No. 2918), “some part of its weight” (49 CFR 390.5T). Of the texts compared, only the British one puts a figure on it: “not less than 20% of the weight of its load is borne by the drawing vehicle” when the trailer is uniformly loaded.

Where the axles are. Regulation (EU) 2018/858 (Article 3(33)) and R.E.3 (2.4.5.1) place them “behind the centre of gravity of the vehicle (when uniformly loaded)”. R.E.3 adds that “One or more of the axles may be driven by the towing vehicle”, and the British semi-trailer includes a vehicle “which has some or all of its wheels driven by the drawing vehicle”. The Glossary for transport statistics describes the semi-trailer as a “Goods road vehicle with no front axle designed in such way that part of the vehicle and a substantial part of its loaded weight rests on a road tractor”.

R.E.3, a consolidated resolution of the UNECE World Forum WP.29, notes that its definitions of paragraph 1 are according to the Vienna Convention on Road Traffic of 8 November 1968 and the European Agreements supplementing it, except the road tractor, taken from the OECD “Glossary of statistical terms”.

Semi-trailer, trailer and drawbar trailer

Most of the texts treat the semi-trailer as a kind of trailer: “a trailer in which the axle, or axles are positioned behind the centre of gravity” (Regulation (EU) 2018/858, Article 3(33)); a trailer “includes semi–trailers” (R.E.3, 1.5); the semitrailer is listed under “Trailer includes” (49 CFR 390.5T); “a trailer which is constructed or adapted to form part of an articulated vehicle” (British regulation 3); and in Law No. 2918 the yarı römork is a römork, the unpowered vehicle drawn by a motor vehicle to carry people or goods. Directive 96/53/EC defines a trailer “excluding semi-trailers”, and Spain’s Reglamento General de Vehículos defines the remolque and the semirremolque in separate entries. In everyday Turkish a yarı römork is a dorse.

Annex I, Part C of Regulation (EU) 2018/858 gives each type of trailer a two-letter code. The definitions separate them by their axles, by the vertical load on the towing vehicle and by the coupling; DF and DG also have a fifth wheel at the rear.

R.E.3 sorts trailers of categories O2, O3 and O4 into three types: the semi-trailer, the full trailer, which “transmits no significant static load to the towing vehicle”, and the centre-axle trailer, with the same limit of 10 per cent of the trailer’s maximum mass or 1,000 daN as code DC (2.4.5). Spain’s remolque completo transmits no significant load to the towing vehicle, “(menos de 100 kg.)” (less than 100 kg). In the United States no part of a full trailer’s weight, “except for the towing device”, rests on the towing vehicle, and “A semitrailer equipped with an auxiliary front axle (converter dolly) shall be considered a full trailer” (49 CFR 390.5T). Directive 96/53/EC calls a motor vehicle with a trailer a road train, as Spain does with the tren de carretera, and code BD of Regulation (EU) 2018/858, the road tractor, is built “to tow trailers other than semi-trailers”. The bodies built on semi-trailers, from curtainsiders to low loaders, are described in trailer types explained.

The coupling: king pin and fifth wheel

Code DA makes the coupling part of the definition: “The coupling to be used for a vehicle combination shall consist of a king pin and a fifth wheel”. Code DE rules that coupling out for a rigid drawbar trailer, and codes DF and DG put a fifth wheel at the rear of a trailer. The figures that decide whether a tractor can couple to a given semi-trailer, such as the D value, the imposed load and the coupling height, are covered in kingpin and fifth wheel.

How the tractor unit and the semi-trailer are classified by mass

Regulation (EU) 2018/858 (Article 4(1)) sorts motor vehicles designed and constructed primarily for the carriage of goods by maximum mass into categories N1 (not exceeding 3.5 tonnes), N2 (exceeding 3.5 tonnes but not exceeding 12 tonnes) and N3 (exceeding 12 tonnes), and trailers into O1 (not exceeding 0.75 tonnes), O2 (exceeding 0.75 tonnes but not exceeding 3.5 tonnes), O3 (exceeding 3.5 tonnes but not exceeding 10 tonnes) and O4 (exceeding 10 tonnes); R.E.3 sets the same limits in kilograms. For a tractor unit, “the maximum mass to be considered for classifying the vehicle shall include the maximum mass of the semi-trailer borne by the fifth wheel coupling” (Annex I, point 2.2.1). For a semi-trailer, the mass that counts is “the maximum mass transmitted to the ground by the wheels of an axle or group of axles when coupled to the towing vehicle” (point 2.2.3). In R.E.3 (2.3.4.1), the mass to be considered for classifying a tractor for semi-trailer is the mass of the tractor in running trim, increased by the mass corresponding to the maximum static vertical load transferred to it by the semi-trailer and, where applicable, by the maximum mass of the tractor’s own load.

US federal rules group vehicles by gross vehicle weight rating in Table II of 49 CFR 565.15, whose top group, Class 8, is “Greater than 14968 kg. (33,001 lbs. and over)”; the ratings are explained in GVWR, GCWR and GAWR. How much a semi truck may weigh on the road and how long it may be are set by other rules: for the European Union in truck weights and semi-trailer dimensions, for the United States in how much a semi-truck weighs and semi-truck dimensions. The definitions of logistics and of the carrier and forwarder roles in road freight are compared in what is logistics.