# Data Flow Diagrams - Difference between Lines and Arrows

I'm currently working with Visio to create Data Flow Diagrams for a System Analysis and Design class but I'm unsure what the difference between `------` and `------>` is. I can connect 2 shapes together with a line (process, entity, data store) but does the single line connecting the two mean data flow? Do I need to explicitly use the data flow arrow to show which way data is flowing?

(There doesn't seem to be tags for this topic, maybe im in the wrong place?)

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## migrated from cs.stackexchange.comSep 23 '12 at 15:37

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I'm told that this is a diagram notation used in the industry, but not a notation commonly used by computer scientists. So I'm bouncing this question to a site frequented by programmers. – Gilles Sep 23 '12 at 15:37

yes,single line having arrow connecting the two enties is data flow
arrow is used for show data flow between two entities & only line but two lines in parallel is used for indicating data stores

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This fails to state what the difference between the line with the arrow and the line without the arrow is. – Dave Clarke Sep 23 '12 at 8:15
@DaveClarke:arrow is used for show data flow between two entities & only line is used for indicating data stores – Ravindra Bagale Sep 23 '12 at 8:21
I was actually Looking at This Where Rectangles are Entities, Arrows are Data Flow, Rounded Rectangles are Processes. I didn't know there were more than 1 DFD Standard. – Howdy_McGee Sep 24 '12 at 0:57
@Howdy_McGee: yes some symbols are depriciated, different standards uses different symbols.e.g. for data store some uses simply two parallel lines, some uses halfly bounded rectangles – Ravindra Bagale Sep 24 '12 at 11:27
This is not quite correct. A single line indicates a data store. Two lines in parallel is just a convenience, allowing arrows to come in from two directions without one of them going over the text describing the data store. See "Structured Analysis and System Specification", by DeMarco, which is THE defining document for Data Flow Diagrams. – John R. Strohm Sep 24 '12 at 12:09

A single line is not part of Data Flow Diagramming standard symbols. An Arrow is. Visio is a drawing tool, it does not enforce the diagramming standards.

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I only have a limited understanding of DFD, but I believe this is correct. You have circles, rectangles, two parallel lines, and arrowed lines as your only symbols - a relatively small vocabulary. Each of these symbols is than annotated with text to identify what component of the system is represents. – Thomas Owens Sep 23 '12 at 15:54

A line without an arrow (usually a dashed line) is a trust boundary and is usually drawn over and perpendicular to one or more arrowed lines. For example, it may indicate that entities on one side are on a web page and ones on the other are on the server. This is not a part of the traditional data flow diagrams but is used in threat modelling. See the STRIDE Approach MSDN article for examples.

A line with an arrow shows data flow from one entity to another in the direction indicated by the arrow.

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