35 U.S.C. § 112(b) states that "[t]he specification shall conclude with one or more claims particularly pointing out and distinctly claiming the subject matter which the applicant regards as his invention." And this language is consistent with some of the earliest language of our patent laws.1 Despite the trend of courts looking to the rest of the patent to clarify what the claims entail, the claims are still indisputably the most important part of the patent document. They are "the portion of the patent document that defines the patentee's rights."2 Claims can be analogized to the "metes and bounds" or "fence posts" of a plot of real property (many refer to modern claim drafting as "peripheral" claiming). It is unnecessary to provide every single detail of a given "embodiment" that practices the claims. Rather, one must provide limitations in the claims that clarify (1) what the overall process, machine, article, or composition is, and (2) what distinct limitations thereof are new or "novel."
Types of Patent Claims
35 U.S.C. § 101 sets out the four "statutory classes" of patent claims, stating that "[w]hoever invents or discovers any new and useful process, machine, manufacture, or composition of matter" or any improvement thereof may obtain a patent. A process is an act or series of acts, a machine is a concrete device or combination of devices (often referred to as a "system"), a composition of matter is a composition of two or more substances, and a manufacture (or "article" of manufacture) is a human-made tangible thing made by a combination of raw or prepared materials.3 A claim MUST fall into one, and only one, of the statutory classes to be valid. For instance, if a claim recites both an apparatus and a method of use of the apparatus, it would be found invalid.4
Before discussing several of the more nuanced aspects of patent claim drafting, like enablement and eligibility, I think it is appropriate at this point to give a couple of examples of simple patent claims. For this section, I have chosen patent claims that have been the subject of important litigation to provide a glimpse of how courts deal with the important job of interpreting patent claims.
A. Machine - U.S. Pat. No. 5,575,405
The following claim is for a post-mix beverage dispenser with an associated visual display that provides an optical illusion. See if you can figure out what the optical illusion is.
In a post-mix beverage dispenser of the type having an outlet for discharging beverage components in predetermined proportions to provide a serving of dispensed beverage, the improvement which comprises:
a transparent bowl having no fluid connection with the outlet and visibly containing a quantity of fluid;
said fluid being resistant to organic growth and simulating the appearance of the dispensed beverage;
said bowl being positioned relative to the outlet to create the visual impression that said bowl is the reservoir and principal source of the dispensed beverage issuing from the outlet; and
said bowl and said quantity of fluid visible within said bowl cooperating to create the visual impression that multiple servings of the dispensed beverage are stored within said bowl.
Before we take a closer look at the claim and discuss some of the key features, let's get to the illusion. The claim is for a drink dispenser with a hidden compartment and a visible display with a separate compartment. The hidden compartment actually has the consumable drink, but it is delivered from a spigot that appears to be connected to the visible portion. The machine, a Juicy Whip post-mix beverage dispenser, is popular because it provides consumers with an appealing visual display that mimics a pre-mix beverage dispenser but is conveniently not as difficult to clean as an actual pre-mix beverage dispenser. When Juicy Whip accused Orange Bang of infringing the patent, the Federal Circuit had to determine whether the illusion had beneficial utility (a requirement under 35 U.S.C. § 101).5
While the Juicy Whip case is fascinating, let's discuss some of the more basic claim elements. For one, notice that the claim is a single sentence separated by semi-colons. This has long been the standard custom for patent claims and is suggested by the Manual of Patent Examining Procedure (the "MPEP" or "M.P.E.P.").6 While it may lead to some of the longest, most obtuse sentences in the history of the English language, it provides consistency to the practice of claim drafting. Next, notice that the claim begins with a "preamble," which includes "a general description of all the elements or steps of the claimed combination which are conventional or known."7
After the preamble is a list of elements, or "limitations" that signify the invention's essential aspects. To successfully enforce a patent against an infringer, the patentee must show that the accused device contains every limitation in the claim. After the preamble and before the limitations, a claim must include a transition phrase. The transition phrase is crucial because it tells the reader whether the claim is "open" or "closed." Open claims use the transitional phrase "comprising," and cover any embodiment that includes the limitations as well as any other combination of elements. Closed claims use the term "consisting of" and cover only those accused devices that contain the limitations of the claims without more. So obviously, an "open" claim is far broader than a "closed" counterpart.
You may notice that some of the limitations begin with "a" while others begin with "said" or "the." Limitations in claims must be introduced with "a" or "an" before being referred to with "said" or "the" so that each term in the claim has a clear "antecedent basis." Many modern claim drafters have ditched the archaic use of "said" and use "the," going with the trend across practice areas to modernize legal documents and to rid them of "legalese."
 See W. Robinson, Robinson on Patents §§ 504-38 (Hein 1st ed. 1972) (noting that the Patent Act of 1836 stated that a patent applicant "shall also particularly specify and point out the part, improvement, or combination which he claims as his own invention or discovery.").
 Markman v. Westview Instruments, Inc., 517 U.S. 370, 372 (1996).
 Robert C. Faber, Faber on Mechanics of Patent Claim Drafting, 5 (6th ed. 2008).
 Id. at 6 (citing IPXL Holdings, L.L.C. v. Amazon.com, Inc., 430 F.3d 1377 (Fed. Cir. 2005)).
 Juicy Whip, Inc. v. Orange Bang, Inc. 185 F.3d 1364 (Fed. Cir. 1999) (holding that "[t]he fact that one product can be altered to make it look like another is in itself a specific benefit sufficient to satisfy the statutory requirement of utility.").
 See M.P.E.P. § 608.01(m) ("While there is no set statutory form for claims, the present Office practice is to insist that each claim must be the object of a sentence starting with 'I (or we) claim,' 'The invention claimed is' (or the equivalent).").
 Id. at M.P.E.P. § 608.01(i).