1.  Field of the Invention
This invention pertains generally to geometrical instruments, and more specifically to straight-edge rules that include an index for subdividing the scale, commonly referred to as a vernier. 
2.  Description of the Related Art
Devices for measuring distances and geometries of objects are quite old, dating back to prehistoric times.  The early devices were designed to measure using units associated with commonly available objects, such as forearms, hands and feet.  Distances were generally defined in whole units and fractions were used only infrequently, being more difficult to calculate and determine.  Commonly available objects were identified that could be used for smaller and larger measurements, which reduced the need for fractions or large values.  Eventually, whole unit measurements gave way to fractional divisions of existing units, such as the division of one foot into twelve equal inches.  Inches were further divided into fractions by halves, into one-half inch, one-quarter inch, one-eighth inch and smaller divisions.  For the purposes of this disclosure, fractional units are defined as this division of whole units by multiples of two, and will be specifically understood to include these units of half, quarter, eighth, and so forth. 
As time has passed from those early days, so has the development of technology.  Advances in technology requiring smaller, more durable, longer life devices have been accepted as commonplace, yet the foundation required for these advances is often misunderstood or taken for granted. 
To manufacture smaller components, components at greater yield and lower prices, or components capable of special performance or reliability requires the ability to introduce precision into the tools, machines and processes are used to produce the resulting components.  These tools, machines and processes must have the same or better precision than that of the finished component.  Yet, determining the precision of the tools, machines and processes requires the use of measuring devices capable of measuring widely diverse devices and objects.  The measuring devices must, once again, have precision equal or greater than the precision required of the tools.  The precision must start with the instruments used to measure other devices and objects. 
In modern production, these measurements are often more precise than would be readily identified by fractions of an inch, even though many measurements are still specified based upon the fractional system.  For example, a hole might be identified as having a one-half inch diameter, but precision may be specified to the nearest hundredth of an inch.  Another dimension may be specified as having an outside diameter of 0.625 inches, which is five-eighths of an inch, with a tolerance of plus or minus five thousandths of an inch.  These types of mixed fractional and decimal dimensions are commonplace in a manufacturing environment today. 
Unfortunately, the development of instruments that readily measure and evaluate these fractional and decimal dimensions has not kept pace with the changing needs of the manufacturing environment.  Calculators have been developed that will perform conversions between decimal and fractional formats.  However, these calculators are not well suited to a manufacturing environment, and are prone to being destroyed by contamination, spills or accidental impact with tools, equipment or the shop floor.  They must also be carried about to be of any real use on the shop floor, therefore requiring yet another pocket or pouch.  Furthermore, the use of a separate device from the measuring instrument requires a separate step of keying information into the calculator, taking valuable time and introducing the possibility of keying errors.  Since there is no direct visual feedback of proportions or relationships between the units of measure, these mistakes may easily go unnoticed until a later time, when the cost of the error is amplified by production of many bad parts. 
In the prior art, measuring devices frequently have fairly well developed attachments which allow the measurement of a wide and diverse set of components.  Typically, these measurements will include inside and outside diameters, elevations, thickness, gap and other similar measurements.  Unfortunately, and in spite of their flexibility at measuring diverse components, these instruments are calibrated to either fractional or decimal measuring, but do not provide the ready ability to convert from one format to another. 
U.S. Pat.  No.  897,437 to Watson is representative of early versions of measuring instruments having both coarse and fine measurement which are capable of measuring a variety of dimensions.  A straight rule is provided that has standard graduations marked thereon.  Onto the rule there are clamped several arms which extend perpendicularly from the rule.  These arms enable the measurement of diverse dimensions by allowing a part to be placed between the arms, to measure thickness or outside diameter, or allowing the arms to be placed within the part, such as for inside diameter.  While these types of instruments have met with great success in the trade because of their tremendous versatility in taking measurements of many different types, several deficiencies are noteworthy.  In particular, one or both of the adjustable arms cover a large number of graduations on the rule.  Since most rules use larger and smaller marks to distinguish different graduations, covering up adjacent marks makes it much more difficult to discern quickly and accurately the particular graduation that is exposed.  In addition, the precision of these devices is limited to the smaller sizes of graduations that may be placed upon the scale.  While in theory a very large number of such graduations are possible, attempting to place them on the scale and still remain legible and useful is not practically possible.  In practice, even scales divided to a sixteenth of an inch become visually "busy", and these finer scales require more time to accurately discern the measurement. 
A second limitation is in the ability to quickly convert from fractions to decimals, such as when the part is specified by a combination of fractional and decimal units. 
A third limitation arises from the fact that the alignment for measuring must occur between two perpendicular planes.  The vertical edge of a movable body must be visually aligned with a horizontal Graduation mark.  Because the vertical edge and horizontal mark are not co-planar.  and are furthermore not of similar width and dimension, accurate correlation between the two different structures is difficult.  As a result, any precision beyond the usual sixteenth of an inch is increasingly difficult. 
In order to overcome the human visual limitation of reading closely spaced graduations, vernier scales were developed such as disclosed by Homan in U.S. Pat.  1,602,490; Berger in U.S. Pat.  No.  1,888,305; and Huffman in U.S. Pat.  No.  1,888,597.  The graduations on the vernier align with the main scale only at the appropriate fractional point of measurement.  For example, in the decimal system of measurement, a vernier will divide into ten equal spaces the distance occupied by nine spaces on the scale.  When the first vernier graduation mark aligns with a graduation mark on the main decimal scale, the vernier will indicate one-tenth the smallest main scale division.  So, carrying this example further, if the main scale is divided into tenths of an inch, the vernier will be calibrated to identify hundredths of an inch without visually cluttering the main scale.  This concept has also been widely adapted into the measuring instruments of the prior art, since they quickly advanced the resolution of these versatile instruments. 
Alternatives to the vernier have been proposed, such as the sawtooth line of Clay in U.S. Pat.  No.  4,607,436.  However, these alternatives have not proven to offer sufficient benefit in reading the scale with precision for most applications.  Furthermore, these scales are more difficult to produce with the intended accuracy. 
There is a definite need to convert readily between fractional and decimal units, without the need for resorting to special calculators or extremely expensive and complicated devices.  This need has not been satisfied by existing instruments.