Section 18. All taxable personal estate within or without the commonwealth shall be assessed to the owner in the town where he is an inhabitant on January first, except as provided in chapter sixty-three and in the following clauses of this section:

First, All tangible personal property, including that of persons not inhabitants of the commonwealth, except ships and vessels, shall, unless exempted by section five, be taxed to the owner in the town where it is situated on January first. Ships and vessels, except those used in or designed for use in carrying trade or commercial fishing, shall be taxed to the owner as of July first in the town where it is habitually moored or docked, otherwise where it is principally situated during the calendar year.

Second, Machinery employed in any branch of manufacture or in supplying or distributing water, including machines used or operated under a stipulation providing for the payment of a royalty or compensation in the nature of a royalty for the privilege of using or operating the same, and all tangible personal property within the commonwealth leased for profit, or, in the case of business corporations subject to tax under section 39 of chapter 63, machinery used in the conduct of the business, shall be assessed where such machinery or tangible personal property is situated to the owner or any person having possession of the same on January first.

Second A, For the purposes of this clause the following words shall have the following meanings:—

''Situated'', repeated and regular use of a machine or type of machinery in a city or town in the conduct of the corporation's business. If machinery is regularly used in a city or town as of January first, it shall be deemed situated in that city or town regardless of its physical location on January first.

''Fair cash value'', the machinery's actual fair cash value or, in the case of machinery which is part of a larger fleet of substantially similar machinery, the average fair cash value of the machines in that portion of the fleet physically located in a city or town during the year.

''Fairly apportioned'', allocated so as to reflect only the amount of time during which property was physically located in a city or town, according to records kept by the corporation in the regular course of its business.

Any machinery used in the conduct of its business, and owned or in the possession of a business corporation subject to tax under section 39 of chapter 63, which machinery is designed or used for the transportation of persons or goods within the commonwealth or interstate, including but not limited to: aircraft frames, aircraft engines, and aircraft avionics, shall be assessed as provided by this clause.

The assessed value of such machinery shall be its fair cash value fairly apportioned. Such apportionment shall be determined, for each type of machinery used in the city or town, by multiplying the fair cash value of such machinery by a figure representing the percentage of time the machinery or substantially similar machinery was physically located in a city or town during the year. Said figure may be based on one or more representative periods, to be at least one week each, which fairly reflect the typical amount of time each type of machinery is physically located in a city or town during that year.

The commissioner may issue regulations and guidelines for the assessment of such machinery which shall ensure that no tax shall be unfair, unreasonable or a burden on interstate commerce. Said guidelines may classify such machinery according to its type or the nature of its use, determine the basis for depreciation, set a maximum limit on the amount of time any machine or type of machine shall be considered in a city or town, or otherwise regulate the assessment and taxation of such machinery consistent with the purposes of this clause.

The provisions of this clause shall not be applicable to the property of foreign corporations chartered outside of the United States, nor to machinery the assessment of which is specifically provided for elsewhere in the General Laws or which is made exempt from the provisions of this clause by section forty-nine of chapter ninety.

Third, Personal property of deceased persons, before the appointment of an executor or administrator, shall be assessed in general terms to the estate of the deceased, and the executor or administrator subsequently appointed shall be liable for the tax so assessed as though assessed to him.

Fourth, Personal property of joint owners or tenants in common, other than partners, may be assessed to one or more of such owners, and any person so assessed shall be liable for the whole tax.

Fifth, Underground conduits, wires and pipes laid in public ways, except such as are owned by a street railway company, and poles, underground conduits and pipes, together with the wires thereon or therein, laid in or erected upon private property or in a railroad location by any corporation, except poles, underground conduits, wires and pipes of a railroad corporation laid in or erected upon the location of such railroad, and except poles, underground conduits, wires and pipes laid in or erected upon any right of way owned by a street railway company, shall be assessed to the owners thereof in the towns where laid or erected. Poles, underground conduits, wires and pipes of telecommunications companies laid in or erected upon public or private ways and property shall be assessed to their owners in the cities or towns where they are laid or erected. For purposes of this clause, telecommunications companies shall include cable television, internet service, telephone service, data service and any other telecommunications service providers.

Sixth, Partners, whether residing in the same or different towns, shall be jointly taxed under their firm name, for all tangible personal property belonging to the partnership, except ships and vessels used in or designed for use in carrying trade or commercial fishing, in the place where such property is situated. Each partner shall be liable for the whole tax.

Seventh, Ships or vessels, other than yachts and other pleasure craft, owned by a partnership, shall be assessed to the several partners in their places of residence, if within the commonwealth, proportionally to their interests therein; but the interests of the several partners residing without the commonwealth shall be assessed to the partnership in the place where its business is carried on.