PL/I is Algol-style block-oriented language : Programs can complied separately and consist of exception blocks (stopped co-routines), regular blocks (begin blocks), statements and subroutines. Procedures can be internal or compiled separately. This type of structure allows the programmer to produce highly-modular applications. 

PL/1 was the first popular language that has build-in preprocessor : This feature was later copied (and abused) in C.

A very neat generalization of Fortran's ability to use separately compiled units. Process statement and external MEMORY allocation class.

It defined data types for an abstract generalized computer without regard for actual hardware. PL/1 was the first general purpose language with usable string datatype. It also has variable length bit strings.

One of the first general purpose language that has pointers as first class language feature and BASED templates for them with arbitrary data structure.

PL/I contained very powerful set of control structures control structures which in some ways remains unsurpassable and that is FAR superior to what C language GOT (despite C being a DERIVATE of PL1). Of mainstream languages only Perl can compete with PL/1 as for richness and flexibility of control structures.

PL/I supports arrays, structures, unions, arrays of structures or unions, structures or unions of arrays, and combinations thereof.

PL/1 concept of area and ability to allocate storage in particular area along with multi-entry procedures was the construct that allow OO-style programming without typical for OO overhead.

PL/1 was used in Multics as a system programming language, one of the first attempt to write OS in HIGH level language (that attempt inspired Unix and C has by-and-large PL/1 semantic and structure).

PL/1 compilers used to represent to edge of compiler development with IBM optimizing and debugging compliers for many years representing the top of the class. PC/C compiler (which was developed in Cornell and compiled large subset of PL/1) was and remains one of best compiler for educational purposes that I ever saw. They still look amazing even now, half a century s after their initial release, especially in diagnostic messages quality.

Multi tasking was implemented by PL/I F, the Optimizer and the newer AIX and Z/OS compilers. It comprised the data types EVENT and TASK,the TASK-option on the CALL-statement (Fork), the WAIT-statement (JOIN), the DELAY(delay-time), EVENT-options on the record I/O statements and the UNLOCK statement to unlock locked records on EXCLUSIVE files. 

PL/I is Algol-style block-oriented language : Programs can complied separately and consist of exception blocks (stopped co-routines), regular blocks (begin blocks), statements and subroutines. Procedures can be internal or compiled separately. This type of structure allows the programmer to produce highly-modular applications. 

PL/1 was the first popular language that has build-in preprocessor : This feature was later copied (and abused) in C.

A very neat generalization of Fortran's ability to use separately compiled units. Process statement and external memory allocation class.

It defined data types for an abstract generalized computer without regard for actual hardware. PL/1 was the first general purpose language with usable string datatype. It also has variable length bit strings.

One of the first general purpose language that has pointers as first class language feature and BASED templates for them with arbitrary data structure.

PL/I contained very powerful set of control structures control structures which in some ways remains unsurpassable and that is far superior to what C language got (despite C being a derivate of PL1). Of mainstream languages only Perl can compete with PL/1 as for richness and flexibility of control structures.

PL/I supports arrays, structures, unions, arrays of structures or unions, structures or unions of arrays, and combinations thereof.

PL/1 concept of area and ability to allocate storage in particular area along with multi-entry procedures was the construct that allow OO-style programming without typical for OO overhead.

PL/1 was used in Multics as a system programming language, one of the first attempt to write OS in high level language (that attempt inspired Unix and C has by-and-large PL/1 semantic and structure).

PL/1 compilers used to represent to edge of compiler development with IBM optimizing and debugging compliers for many years representing the top of the class. PC/C compiler (which was developed in Cornell and compiled large subset of PL/1) was and remains one of best compiler for educational purposes that I ever saw. They still look amazing even now, half a century s after their initial release, especially in diagnostic messages quality.

Multi tasking was implemented by PL/I F, the Optimizer and the newer AIX and Z/OS compilers. It comprised the data types EVENT and TASK,the TASK-option on the CALL-statement (Fork), the WAIT-statement (JOIN), the DELAY(delay-time), EVENT-options on the record I/O statements and the UNLOCK statement to unlock locked records on EXCLUSIVE files.